The phosphotransferase system (PTS) is a well-char-acterized group translocation system present in many bacteria. It uses energy from phosphoenolpyruvate (PEP), an intermediate in glycolysis, to attach a phos-phate to specifi c sugars during their transport into the cell. Glucose, for example, is converted during transport to glucose 6-phosphate. The system has a modular design that accommodates diff erent substrates. Some protein elements are used by all sugars transported by the PTS, while other elements are unique to a given carbohydrate (Fig. 1A).

Common elements include Enzyme I (PtsI), which is associated with the membrane at the cell poles (Fig. 1B), and a histidine-rich protein called HPr (PtsH), which interacts with Enzyme I (EI) and diffuses in the cytoplasm. Enzyme I strips the high-energy phosphate from PEP and passes it to HPr, which in turn delivers the phosphate to various substrate-specific transport pro-teins called Enzyme II that are distributed around the cell periphery (Fig. 1C). A typical Enzyme II comprises three domains (A, B, and C) that may be fused together as a single polypeptide or assembled in a variety of com-binations. Regardless of the configuration, the phos-phorylated HPr (HPr-P) transfers its phosphate to the Enzyme IIA domains/proteins, which relay the phos-phate to their cognate Enzyme IIB domains/proteins. Enzyme IIB finally delivers the phosphate to the spe-cific sugar that has been transported into the cell by the Enzyme IIC domain embedded in the cytoplasmic mem-brane. Glucose, for example, is transported by Enzyme IIC and converted to glucose 6-phosphate by Enzyme IIB. In Chapter 9, we discuss how this physiological sys-tem impacts the genetic control of many other systems.

eTOPIC 4.1�Transport by Group Translocation: The Phosphotransferase System

FIGURE 1 ■ Group translocation: the phos-photransferase system (PTS) of E. coli. A. The phosphate group from phosphoenolpyruvate (PEP) is ulti-mately passed to the substrate during transport. The com-mon elements of the PTS are Enzyme I (Pts I) and HPr (histidine-rich protein; PtsH). Each Enzyme II is specific for a given substrate and consists of modular components. Enzyme II for mannitol is one protein with three domains: A, B, and C. Enzyme II for glucose is really two proteins: one protein contains the A domain, and the B and C domains are joined to form the second protein. Enzyme II for mannose is designed with the opposite arrangement; namely, its A and B domains are fused into one protein, whereas the membrane protein is simply the C domain. B. Escherichia coli expressing Enzyme I (EI) fused to a fluorescent protein called mCherry. Enzyme I localizes to the cell poles. C. An Enzyme II protein (BgIF) fused to green fluorescent protein distributes around the entire membrane. Source: L. Lopian et al. 2010. EMBO J. 29:3630–3645.

A.

B. C.

2. Substrates are transformed by phosphorylation during transport.

PEP Pyruvate

Cytoplasm

1. Phosphate from PEP is passed along common elements of the PTS to the modular Enzyme II and ultimately to the substrate.

Enzyme II

Cell membrane Extracellular fluid

EI-mCherry BglF-GFP

Mannitol

Glucose

Mannose

Mannitol 6-P

Mannose 6-P

Glucose 6-P

Enzyme II components are modular.

Cell pole

3. Enzyme IIC transportssubstrate across the cell membrane. Enzyme IIB transfers phosphate to the sugar transported across the cell by IIC.

HPr

HPr

EI

EI

IIC

IIBIIA

IIBIIA

IIBIIA

IIC

IIC

P

P

P P

PP

P P

2 μm 2 μm

Enzyme I

L. L

OPI

AN E

T AL

. 20

10. E

MBO

J. 2

9:36

30–3

645

L. L

OPI

AN E

T AL

. 20

10. E

MBO

J. 2

9:36

30–3

645

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