Catalytic alpha subunit of the rod-specific cGMP phosphodiesterase (PDE6) complex, which hydrolyzes 3',5'-cyclic GMP in the phototransduction cascade. The PDE6 holoenzyme consists of two catalytic subunits (PDE6A and PDE6B) and two inhibitory gamma subunits (PDE6G) (PubMed:20940301). Light-activated GNAT1 relieves gamma subunit-mediated inhibition, enabling the catalytic subunits to hydrolyze cGMP and thereby mediate visual signal transduction and amplification. Decreased cytosolic cGMP levels result in closure of cGMP-gated cation channels at the plasma membrane, leading to rod photoreceptor hyperpolarization (By similarity). . This is the function of Ensembl gene identifier ENSG00000132915 (PDE6A, phosphodiesterase 6A).