Characterization of the tyrosine phosphorylation and distribution of dystrobrevin isoforms.

Dystrobrevin, a member of the dystrophin family of proteins, was initially identified as a major tyrosine phosphorylated synaptic protein in the electric organ of Torpedo californica. In this paper, we show that the major sites of tyrosine phosphorylation of Torpedo dystrobrevin are within its C-terminus, on Tyr-693 and Tyr-710. Cloning of the mammalian homologue of dystrobrevin has recently shown that this phosphotyrosine containing tail, or PYCT, is subject to alternative splicing. To compare the expression and distribution of PYCT- and PYCT+ splice variants, we generated antibodies against different regions of dystrobrevin. Here we show that the PYCT- isoform of 62 kDa is expressed at high levels in all tissues examined. In contrast, PYCT+ isoforms are expressed primarily in brain and muscle, where they are concentrated at synapses. Moreover, PYCT+ isoforms associate more tightly with the membrane and with syntrophin, another synaptically enriched protein. These results suggest that PYCT+ isoforms of dystrobrevin are specialized components of the dystroglycan complex which render the complex sensitive to regulation by tyrosine kinases.