User:Cellular Biochemistry II/Article 11

Introduction
Cytosolic regulator of adenylyl cyclase (CRAC) is a Pleckstrin homology domain (PH domain) containing protein regulating chemotaxis and activating adenylyl cyclase in Dictyostelium discoideum.

Whole sequence database blasts using NCBI BLAST2.2.19 searching CRAC (UniProt ID: P35401) in UniProt database (release 16.6.2009) did not reveal a homologous protein in other species besides Dictyostelium discoideum.

Structure
CRAC (Uniprot accession number: P35401) is a 78.6 kD protein encoded by the dagA gene, which is located on chromosome 4. CRAC contains a pleckstrin-homology domain (PH domain) at the amino acid residues 22-122. Within the pleckstrin homology domain, the amino acid residue 42, an arginine, has been shown to be essential for the binding of 3-phosphoinositides to CRAC . In addition, CRAC contains a Poly-Thr and a Poly-Ser region.

Regulation
In Dictyostelium cells, cAMP acts as a chemoattractant and thereby leads to aggregation of individual amoebae into multicellular mounds . A cell that has received the chemotactic signal can relay the signal to surrounding cells, which induces the organization of single cells into an aggregation stream. The extracellular signal is sensed by the surface cAMP receptors cAR1, which in turn activate G-proteins . Upon activation, the βγ-subunit transduces the signal via Phosphoinositide 3-kinase (PI3K) to CRAC . PI(3,4,5)P3 and PI(3,4)P2, phosphorylated products of PI3K, can bind to the pleckstrin homology domain in vitro. This binding recruits CRAC to the leading edge of the plasma membrane when cAMP is secreted .

Function


Clustering Dictyostelium cells secrete cAMP which leads to the production of more cAMP and chemotaxis . CRAC protein acts downstream of the cAMP receptor cAR1 and mediates chemotactic behaviour via adenylyl cyclase stimulation in the back of the dictyostelium cell. The mechanism by which CRAC, located at the leading edge of the cell, leads to adenylyl cyclase activation at the rear end of the cell has not been elucidated yet . Adenylyl cyclase activity increases the concentration of the second messenger cAMP and thereby signals to the posterior cell that will consequently line up in the aggregation stream and propagate the signal to the next cell (Fig.1). Those two functions of CRAC, regulation of chemotaxis and activation of adenylyl cyclase, are independent of each other and mediated by the C-terminal and the PH- domain containing N- terminal, respectively . Additionally, CRAC, more precisely its C-terminal domain, is involved in completion of fruiting body morphogenesis and spore formation in the developing mound .