Binding of laminin alpha1-chain LG4-5 domain to alpha-dystroglycan causes tyrosine phosphorylation of syntrophin to initiate Rac1 signaling.
Previously, a signaling pathway was described [Oak, Zhou, and Jarrett (2003) J. Biol. Chem. 278, 39287-39295] that links matrix laminin binding on the outside of the sarcolemma to Grb2 binding to syntrophin on the inside surface of the sarcolemma and by way of Grb2-Sos1-Rac1-PAK1-JNK ultimately results in the phosphorylation of c-jun on Ser(65). How this signaling is initiated was investigated. Grb2-binding to syntrophin is increased by the addition of either laminin-1 or the isolated laminin alpha1 globular domain modules LG4-5, a protein referred to as E3. This identifies the LG4-5 sequences as the region of laminin responsible for signaling. Since laminin alpha1 LG4 is known to bind alpha-dystroglycan, this directly implicates alpha-dystroglycan as the laminin-signaling receptor. E3 or laminin-1 increase Grb2-binding and Rac1 activation. In the presence of E3 or laminin-1, syntrophin is phosphorylated on a tyrosine residue, and this increases and alters Grb2 binding. The alpha-dystroglycan antibody, IIH6, which blocks binding of laminins to alpha-dystroglycan, blocks both the laminin-induced Sos1/2 recruitment and syntrophin phosphorylation, showing that it is alpha-dystroglycan binding the LG4-5 region of laminin that is responsible. The C-terminal SH3 domain of Grb2 (C-SH3) binds only to nonphosphorylated syntrophin, and phosphorylation causes the Grb2 SH2 domain to bind and prevents SH3 binding. Syntrophin, tyrosine phosphate, beta-dystroglycan, and Rac1 all co-localize to the sarcolemma of rat muscle sections. A model for how this phosphorylation may initiate downstream events in laminin signaling is presented.
Department of Biochemistry, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA.
Yan Wen
YW
Zhou
Donald B
DB
Thomason
Donald
D
Gullberg
Harry W
HW
Jarrett
D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D049030:Dystroglycans / Q000276:immunology / Q000378:metabolism*; D049029:Dystrophin-Associated Proteins / Q000378:metabolism*; D051380:GRB2 Adaptor Protein / Q000378:metabolism; D007797:Laminin / Q000378:metabolism*; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D032446:Myoblasts / Q000378:metabolism; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D011817:Rabbits; D051381:Rats; D012508:Sarcolemma; D015398:Signal Transduction / Q000187:drug effects; D020664:Son of Sevenless Proteins / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism*; D020830:rac1 GTP-Binding Protein / Q000502:physiology*
D049029:Dystrophin-Associated Proteins; D051380:GRB2 Adaptor Protein; C495949:Grb2 protein, mouse; D007797:Laminin; D020664:Son of Sevenless Proteins; C121346:laminin 1; C091824:syntrophin; D049030:Dystroglycans; D014443:Tyrosine; D020830:rac1 GTP-Binding Protein