[Structure and physiological function of connexin proteins].

Connexins are integral transmembrane proteins which form specialized hemichannels (connexons) in the plasma membrane. These structures make up gap junctions in adjacent cells which allow for rapid propagation of action potential and slow diffusion of nonorganic ions, secondary messengers, and other small water-soluble molecules (<1.0-2.0 kDa). Connexin proteins are crucial for the formation of gap junctions. Twenty human and 21 murine connexin isoforms (23-64 kDa) have been described so far. Traditional nomenclature in the CxMW format takes into account only the molecular weight of a given connexin. A more recent classification is based on structural gene similarities, their homology and sequence, as well as the length of connexins' cytoplasmic domains. Connexins, as all proteins, have a unique amino-acid sequences and molecular weights and exhibit specific biochemical properties. However, all of them have a common 3-D structure with four hydrophobic transmembrane domains (TM1-TM4), one cytoplasmic (CL) and two extracellular (E1-2) loops, and C- and N-terminal cytoplasmic regions. The cytoplasmic loop and C-terminal regions bind other structural proteins, creating a protein complex crucial for synchronized intercellular communication.