Mating type proteins are sequence specific DNA-binding proteins that act as master switches in yeast differentiation by controlling gene expression in a cell type-specific fashion. Transcriptional activator that induces the transcription of A-specific genes like mating factor ccg-4. Required for mating as an A-cell and for blocking of heterokaryon formation (vegetative incompatibility). Only present in A-cells and in a/A diploid cells. Belongs to the MATALPHA1 family. Do not confuse Mt a-1 with MT A-1; these are two different proteins.