Insulin-induced maturation of Xenopus oocytes is inhibited by microinjection of a Brassica napus cDNA clone with high similarity to a mammalian receptor for activated protein kinase C.

A cDNA clone encoding a WD-40 repeat protein (BGB1) was characterized in Brassica napus L. The clone contained an open reading frame of 327 amino acid residues almost entirely composed of seven segments of WD-40 repeats. Among the WD-40 repeat proteins, BGB1 showed high similarity (63% identity) to a rat intracellular receptor for protein kinase C (RACK1) that functions in the translocation of activated protein kinase C (PKC) from the cytosolic fraction to the membrane fraction. BGB1 also had two sequence motifs involved in binding of RACK1 to PKC. The cDNA clone, when carried in a Xenopus oocyte expression vector and injected into Xenopus laevis oocytes, inhibited insulin-induced maturation of the oocytes, a PKC-mediated pathway, and this inhibition was accompanied by reduction of PKC in the membrane fraction, as in the case of mammalian RACKs. The data show that BGB1 shares some common functional characteristics with the mammalian RACK1 along with the structural similarity, suggesting that a mammalian RACK1-related cellular process might be operating in plants. Southern blot analyses of the genome of B. napus and Arabidopsis thaliana (L.) Heynh. revealed that BGB1-related genes constitute a small multigene family in both species. An approximately 1.4-kb transcript was constitutively expressed in all organs examined.