Alternative splicing as a mechanism for regulating 14-3-3 binding: interactions between hD53 (TPD52L1) and 14-3-3 proteins.

D52 (TPD52)-like proteins are coiled-coil motif-bearing proteins first identified through their expression in human breast carcinoma, which have been proposed to represent signalling intermediates and regulators of vesicle trafficking. D52-like gene transcripts are subject to alternative splicing, with sequences encoding a region termed insert 3 being affected in all three D52-like genes. We have now identified a 14-3-3 binding motif within one of two alternatively spliced exons encoding insert 3. As predicted from the distribution of 14-3-3 binding motifs in four hD52-like bait proteins tested, only a hD53 isoform encoding a 14-3-3 binding motif bound both 14-3-3beta and 14-3-3zeta preys in the yeast two-hybrid system. Since D53 proteins carrying 14-3-3 binding motifs are predicted to be widely expressed, polyclonal antisera were derived to specifically detect these isoforms. Using soluble protein extracts from breast carcinoma cell lines, pull-down assays replicated interactions between recombinant 14-3-3beta and 14-3-3zeta isoforms and exogenously expressed hD53, and co-immunoprecipitation analyses demonstrated interactions between endogenous 14-3-3 and both endogenously and exogenously-expressed hD53 protein. Co-expressed hD53 and 14-3-3 proteins were similarly demonstrated to co-localise within the cytoplasm of breast carcinoma cell lines. These results identify 14-3-3 proteins as partners for hD53, and alternative splicing as a mechanism for regulating 14-3-3 binding.