Human Biochemistry, Vol. 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Membranes: Structure, Assembly, and Functions
Intercellular Junctions and Communication

In a multicellular Organism, A wide variety of intercellular contacts exist. The formation of such contacts requires the direct interaction of the Plasma Membranes of individual Cells. Specialized regions are formed within Cell membranes to facilitate Intercellular Communication. Gap Junctions regulate The transport of ions and small molecules through narrow hydrophilic pores that connect the Cytoplasm of adjacent cells. These pores are formed from subunits, and the corresponding structures are called connexons; their Structure has been investigated using X-ray crystallography. As illustrated in Fig. 42.22, connexons consist of six protein subunits that span the membrane and link up with identical structures in the neighboring cell. Each subunit appears to be a relatively rigid structure, but in response to specific chemical signals, the subunits alter their relative orientation (cf. The behavior of Hemoglobin upon oxygenation; Fig. 6.12) in such a way as to create a central pore approximately 2 nm in diameter. Ions and small molecules apparently pass from one cell to another through this central opening, and this process is regulated.

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Fig. 42.22. A simple model of a connexon, illustrating the transition from the "open" to the "closed" configuration. It is hypothesized that the opening on the cytoplasmic side (top) is closed by the sliding of subunits relative to one another; this is accompanied by a decrease in their tilt and rotation angle relative to the base. The Regions of the subunit embedded in the membrane are shaded. The radial displacement of each subunit on the cytoplasmic side is 0.6 nm with a 5° change in tilt angle, given a subunit length of 7.5 nm. (From Unwin P. N. T., Zampighi G.: STRUCTURE OF THE junction between communicating cells, Nature, 1980, 283, 545.)

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