Mint 3: a ubiquitous mint isoform that does not bind to munc18-1 or -2.

Mint 1 and 2 are proteins that bind to munc18-1, an essential component of the synaptic vesicle fusion machinery, and are detectably expressed only in neurons [Okamoto and Südhof, J. Biol. Chem. 272, 31459-31464 (1997)]. Mint 1 and 2 are composed of a variable N-terminal region that includes a conserved munc18-1-binding site, and a constant C-terminal region that contains one PTB and two PDZ domains. We have now identified a third mint isoform, mint 3. Similar to mint 1 and 2, the C-terminal half of mint 3 is composed of one PTB domain and two PDZ domains. However, in contrast to mint 1 and 2, mint 3 lacks an N-terminal munc18-binding domain and does not interact with munc18-1 in yeast two-hybrid assays. Mint 3 is ubiquitously expressed in all tissues, with lowest levels in brain and testis whereas mint 1 and 2 appear to be brain-specific. Our data suggest that mints form a diverse family of proteins with specialized neuronal and ubiquitous isoforms.