MUSCLE PROTEINS

BY- AMIT SHANKAR SINGH

INTRODUCTION

• The myofiber is the functional unit of skeletal muscle and is a multinucleated tubular structure formed from the fusion of multiple mono-nucleated muscle cells (myoblasts).

• In addition to typical cellular organelles, the cytoplasm (sarcoplasm) of a myofiber contains a regular array of contractile units (sarcomeres) comprised of actin-containing thin filaments and myosin-containing thick filaments that, along with additional structural and regulatory proteins, are arranged longitudinally as myofibrils.

• The peripheral myofibrils are connected to the sarcolemma along the Z-disks via interactions with sub-sarcolemmal protein complexes.

• These structures transmit contractile forces from sarcomeres of one myofiber to another, which prevents sarcolemma ruptures by synchronizing contraction of myofibers within a muscle. Sarcolemma is firmly attached to the basal lamina, With another set of Extracellular matrix proteins providing stability.

CATEGORISATION OF MUSCLE PROTIENS

• MUSCLE PROTIENS CAN BE CATEGORISED ACCORDING TO THERE LOCATION IN MYOBLASTS-

• MYOFIBRIL PROTEINS- Main contractile protiens

• DYSTROPHIN-GLYCOPROTEIN COMPLEX-CONSTITUTED BY SARCOLEMMAL PROTEINS-1. EXTRACELLULAR MATRIX PROTEINS2. TRANSMEMBRANE PROTEINS3. INTRACELLULAR PROTEINS CYTOSKELETAL PROTEINS

• NUCLEAR ENVELOPE PROTIENS

CATEGORISATION OF MUSCLE PROTIENS

MYOFIBRIL PROTEINS

DYSTROPHIN-GLYCOPROTEIN COMPLEX

NUCLEAR ENVELOPE PROTIENS

MYOFIBRIL PROTEINS

MYOSIN

ACTIN

MUSCLE PROTEINS IN MYOFIBRILSLOCATION PROTIEN

Filaments ActinMyosin Troponin I, C, and T SubunitsTropomyosin

Z disk α-actinin DesminVimentinNebulin Titin

M line ParamyosinC-protein M-protein

Z BAND PROTIENS

Dystrophin-glycoprotein complex (DGC)• DGC represents a link between the extracellular matrix and the cytoskeleton and

it is crucial for the structural stability of the plasma membrane.• DGC is a group of peripheral and integral membrane proteins which forms a

mechanical linkage between the F-actin cytoskeleton and the extracellular matrix.

The DGC in skeletal muscle is formed by-1. Cytosolic dystrophin, syntrophins and dystrobrevins; 2. Heavily glycosylated dystroglycan-complex, formed by two subunits, α- and β-Dystroglycan.3. Four sarcoglycans, α, β, γ and δ, plus sarcospan, syntrophins (α-1, β-1 and β-2) and dystrobrevins.

DYSTROPHIN-GLYCOPROTEIN COMPLEX

Dystroglycan• Dystroglycan represents a link between the extracellular matrix and the cytoskeleton and it is crucial for

the structural stability of the plasma membrane.• Dystroglycan is composed of two subunits, α- and β- Dystroglycan. Alpha- Dystroglycan binds to a

number of extracellular matrix molecules, laminin, agrin and perlecan and it interacts tightly with β- Dystroglycan. The cytoplasmic tail of β- Dystroglycan binds dystrophin, caveolin-3 and other cytoplasmic proteins involved in signal trasduction.

α- and β- Dystroglycan

DYSTROPHIN• Dystrophin is an intracellular, rod-shaped protein with four major functional domains. This

protein is localized to the inner surface of the sarcolemma, with a high abundance at Z-disc.• The N terminal and a portion of the middle rod domain interact with the cytoskeletal

filamentous actin (F-actin) which interacts with Z-disc, whereas the C-terminal domain interacts with the cytodomain of transmembraneous β-Dystroglycan along with dystrobrevin and syntrophin protiens of DGC.

DYSTROPHIN• The extracellular domain of β-Dystroglycan binds to

the peripheral membrane protein α-Dystroglycan that interacts with several components of the basement membrane of the skeletal muscle, laminin, perlecan and agrin.

• Therefore through an extensive network of interacting proteins Dystrophin physically couple the sarcolemma with the Z disk of force-generating myofibrils.

• The absence of dystrophin in humans leads to skeletal musle disorganization, sarcolemmal fragility, muscle weakness and necrosis.

Sarcoglycans• Five transmembrane proteins, all expressed primarily in skeletal muscle,

constitute the sarcoglycan family: α (50 kDa, also called adhalin), β (43 kDa), γ (35 kDa), δ (35 kDa) and ε (50 kDa).

• Dystrophin and γ sarcoglycan can interact directly, and δ sarcoglycan appears to be coordinated to the dystroglycan complex.

• Mutations abolishing the expression of any one of the sarcoglycans cause loss of the others from the sarcolemma; the four recessive limb girdle muscular dystrophies 2D, 2E, 2C and 2F are caused by absence of the α, β, γ or δ sarcoglycans, respectively

Desmin• Desmin connects Z-disks of neighboring

myofibrils and anchors myofibrils to intracellular organelles, such as mitochondria and the nucleus, and to the sarcolemma, to maintain a spatial organization between myofibrils and other structural components of the myofiber.

• Mutations in desmin are associated with the autosomal dominant LGMD1D with dilated cardiomyopathy, with characteristic desmin-positive protein aggregates within the sarcoplasma.

Caveolin 3• T-tubules are essential for the coordinated contraction of muscle

fibers. Their composition is similar to that of caveolae, which are plasma membrane invaginations 50–100nm in diameter with the biochemical properties of lipid rafts.

• Caveolin 3 is a muscle-specific isoform of caveolin that associates with developing T-tubules and has been credited with several additional roles in muscle, including inhibition of nNOS at the cell surface.

• Caveolins are small membrane-based molecules that span the membrane twice in a hairpin fashion without a significant extracellular domain.

• Caveolin 3 is partly associated with the DGC by binding to the intracellular domain of beta-dystroglycan.

• However, most caveolin 3 appears not to be directly associated with the complex.

Dystrobrevins and syntrophins • The dystrobrevins and syntrophins are structurally related

to dystrophin's C-terminus, with which they also are associated, thus they are at the same time dystrophin-related and dystrophin-associated proteins.

• Among other interactions, dystrobrevin also appears to bind the intracellular portion of the sarcoglycan complex, thus establishing a link between dystrophin and sarcoglycan. The novel proteins syncoilin and desmuslin bind to dystrobrevin and also to the intermediate filament desmin, thus providing a link to the muscle intermediate filament cytoskeleton.

• Dystrobrevin likely acts with syntrophins to recruit signaling proteins to the DGC (nNOS: nitric oxide synthase)

Sarcospan

• Sarcospan is a 25 kDa membrane protein with four transmembrane domains and intracellular N- and C-termini, a unique feature for transmembrane proteins of the DAPC.

• Expression is seen predominantly in skeletal and cardiac muscle, but shorter isoforms exist in other tissues.

• No human disease is currently known to be• associated with sarcospan deficiency.

Syncoilin• Syncoilin was first identified via its interaction with α-

dystrobrevin in muscle.• Sequence analysis revealed the presence of a unique N-

terminus domain and a coiledcoil domain that is typical of those found in intermediate filament proteins.

• Syncoilin is highly expressed in skeletal, cardiac and smooth muscle at the sarcolemma, Z-lines and neuromuscular junction.

• Through its interaction with desmin, syncoilin is thought to provide a link between the DGC at the sarcolemma and the intermediate filament protein network

Nitric Oxide Synthetase

• The production of nitric oxide (NO) by nNOS is important for increasing local blood flow to match the increased metabolic load of contracting muscles, such as during exercise.

• The presence of nNOS at the sarcolemma is mediated through syntrophin, and it is lost in a number of muscular dystrophies including DMD.

Laminin-2 (Merosin)

• Laminin-2 is composed of α2, β1 and γ1 chains and binds to α-dystroglycan and the α7β1 integrin complex.

• Laminins are thought to form the structural part of the basement membranes along with collagen IV, nidogen and perlecan.

• Mutations of the laminin α2 gene cause severe congenital muscular dystrophy but do not appear to cause damage to the sarcolemma

Dysferlin

• Dysferlin has a membrane-spanning domain, but in contrast to the sarcoglycans, its major proportion is contained within the cell.

• It contains multiple C2 domains, which are thought to play a role in calcium mediated membrane fusion events.

• Thus, a role for dysferlin in membrane maintenance and repair has been suggested.

Collagen type VI

• Collagen type VI is unique among the collagens in that it forms beaded microfibrils that can be found in most extracellular matrices.

• Collagen type VI seems to be in intimate contact with basement membranes through interactions with collagen type IV.

• It also directly binds to cell surface receptors.

MUSCULAR DYSTROPHIES CAUSED BY DEFECT IN MUSCLE PROTIENS

Costameres

• The contractile proteins of myofibers are anchored in Z disks, which are connected to the sarcolemma and the ECM via costameres.

• Costameres are the periodic membrane-associated bands that are rich in vinculin and juxtaposed to the Z-line and M-line.

• At the sarcolemma, these plaques contact the DGC/DAPC and a7b1- integrin for interaction with the ECM. These regions are believed to stabilize domains of the sarcolemma throughout muscle contraction and relaxation.

• In addition, costameres transmit force laterally in both resting and active muscle through the sarcolemma via attachment to the ECM and tendon.

NUCLEAR ENVELOPE PROTEINS

• The nuclear envelope is composed of 2 lipid bilayers, the outer nuclear membrane, which is contiguous with the endoplasmic reticulum, and the inner nuclear membrane.

• Positioned within the inner nuclear membrane are a variety of integral membrane proteins.

• The important ones are- Emerin and lamin A which are expressed in all differentiated cell types, yet these diseases specifically affect only a subset of tissues.

• Thus, it was proposed that Emerin and lamin A have roles in tissue-specific gene expression, cell signaling, or nuclear structure