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 mfa-1. Required for mating as an a-cell, blocking of heterokaryon formation (vegetative incompatibility) and for perithecium induction. Binds in vitro to DNA containing a specific core sequence 5'-CTTTG-3'. Only present in a-cells and in a/A diploid cells. Do not confuse Mt a-1 with MT A-1; these are two different proteins.