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 Biassica napus L. The clone contained an open reading Frame of 327 amino acid residues almost entirely composed of seven segments of WD-4 0 repeats. Among the WD-40 repeat proteins, BGB1 showed high similarit y (63% identity) to a rat intracellular receptor for protein kinase C (RACK1) that functions in the translocation of activated protein kinas e C (PKC) from the cytosolic fraction to the membrane fraction. BGB1 a lso had two sequence motifs involved in binding of RACK1 to PKC. The c DNA clone, when carried in a Xenopus oocyte expression vector and inje cted into Xenopus laevis oocytes, inhibited insulin-induced maturation of the oocytes, a PKC-mediated pathway, and this inhibition was accom panied by reduction of PKC in the membrane fraction, as in the case of mammalian RACK1. The data show that BGB1 shares some common functiona l characteristics with the mammalian RACK1 along with the structural s imilarity, 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-relat ed genes constitute a small multigene family in both species. An appro ximately 1.4-kb transcript was constitutively expressed in all organs examined.