G-Protein

The notation G - protein is simplified for guanine nucleotide - binding protein or GTP - binding protein. G proteins occupy a key position in the signal transduction ( signaling ) between the receptor and second messenger systems. We distinguish between membrane-bound heterotrimeric G proteins and cytosolic so-called small G- proteins.

Heterotrimeric G- proteins

Heterotrimeric G proteins are composed of three subunits ( α, β and γ ) up, the α - subunit has a GDP / GTP - binding domain. In the inactive form of the α subunit has bound GDP and is associated with βγ subunits. A heterotrimeric G- protein can be obtained by a G- protein coupled receptor, which is a GTP exchange factor ( guanine - nucleotide exchange factor GEF) or such binds, are activated. It exchanges the α subunit ( under the influence of the GEF) their bound GDP for GTP, after which the α subunit from the βγ subunits dissociate. The βγ subunits not continue to decay, but represent a functional unit represents the two liberated G protein subunits α and βγ can regulate different effectors. Examples include adenylyl cyclases, G - protein-coupled receptor kinases ( GRKs ) and phospholipases. Of the individual subunits are several isoforms. So far, 21 different α G protein subunits, 5 β subunits and 12 γ subunits are known.

Heterotrimeric G proteins occupy a key position in signal transduction and are responsible for physiological (eg, seeing, smelling, blood pressure regulation, etc.) and pathophysiological effects (eg hypertension, heart failure).

Subfamilies

• Gs proteins: cAMP -dependent reactions
• Gq proteins: phospholipase C -coupled G proteins

Small G proteins ( monomeric )

Small G proteins ( small GTP - ases ) are monomeric GTP -binding proteins with a molecular mass of 20-40 kDa. There are currently over 100 different small G proteins are known to be divided on the basis of phylogenetic similarities and differences in five families: Ras, Rho, Rab, and Ran Sar1/Arf. They are involved in the cell cycle in the regulation of many cellular functions, such as regulation of gene expression ( Ras and Rho ), regulation of the cytoskeleton ( Rho ), the regulation of the vesicular (Rab and Sar1/Arf ) and the regulation of the transport between the nucleus and cytoplasm (RAN). The small GTPases go from an inactive GDP-bound form in the cytosol in an active GTP - bound form to the plasma membrane. They are inhibited by 14-3- 3- proteins.