Immunoreactive glycogen-binding subunit of protein phosphatase-1 in human skeletal muscle

In rabbit muscle, analyzed by Western blot, the glycogen-bound protein phosphatase-1 (PP-1G) is composed of a 37-kilodalton (kDa) catalytic subunit complexed to a 160-kDa glycogen-binding subunit (G-subunit) responsible for the interaction of PP-1G with glycogen. PP-1G has not been characterized in humans. In the present study, G-subunit was identified in human muscle extracts by Western blot using an antibody raised against a sequence (the phosphoregulatory domain) of the rabbit muscle G-subunit. The human G-subunit was also a 160-kDa protein by Western blot. When the G-subunit content of skeletal muscle was quantitated in 17 Pima Indians with a wide range of insulin sensitivities determined during euglycemic clamps, there was a significant negative correlation (r = -0.55; P = 0.02) between the G-subunit content and in vivo insulin-mediated glucose disposal rates. The results suggest that insulin resistance is associated with an increased content and/or immunoreactivity of G-subunit in human muscle.