The skeletal muscle glycogen-binding subunit (G(M)) of protein phospha
tase-1 (PP1) is the founding member of a family of proteins that tethe
r the PP1 catalytic subunit (PP1C) to glycogen and promote the dephosp
horylation of glycogen synthase. A hydrophobic sequence (called here t
he VFV motif) is conserved among G(M), the liver subunit G(L), and the
widely expressed subunits, PTG, R5 and U5. This study analyzed the ro
le of this VFV motif in binding to glycogen and PP1C. Glutathione S-tr
ansferase (GST) fusions with the N-terminal domain of G(M) (GST-G(M(1-
240))) and with the full length R5 protein (GST-RS) both bound to glyc
ogen in a co-sedimentation assay. In contrast, GST itself did not bind
to glycogen. A single residue substitution in GST-G(M(1-240)), F155A,
reduced glycogen binding by 40%. Double residue substitutions V150A/F
155A and F155A/V159A resulted in greater reductions (60-70%) in glycog
en binding, showing these hydrophobic residues influenced the protein-
glycogen interaction. The wild type and V150A/F155A fusion proteins we
re digested by trypsin into the same sized fragments at the same rate.
Furthermore, the wild type and mutated GST-G(M) proteins as well as G
ST-R5 bound equivalent amounts of PP1C, in either pull-down or far-Wes
tern assays. These results demonstrated retention of overall tertiary
structure by the mutated fusion proteins, and indicated that glycogen
and PP1C binding are independent of one another. A 68 residue segment
of R5 encompassing the VFV motif was sufficient to produce glycogen bi
nding when fused to GST. This motif, that is in bacterial and fungal s
tarch metabolizing enzymes, probably has been conserved during evoluti
on as a functional domain for binding glycogen and starch. (C) 1998 Fe
deration of European Biochemical Societies.