title: The Cdk5 activator P39 specifically links muskelin to myosin II and regulates stress fiber formation and actin organization in lens.
abstract: Cyclin dependent kinase 5 (Cdk5), a proline-directed serine/threonine kinase, requires p39 for its enzymatic activity, and is implicated in cytoskeletal organization and contraction in numerous cell types. The C-terminus of p39 binds muskelin, a multi-domain scaffolding protein known to affect cytoskeletal organization, but the mechanisms by which muskelin affects cytoskeletal organization remain unclear. The present study sought to determine whether p39 might serve as an adaptor protein that links muskelin to stress fibers and to investigate the possible biological relevance of such an interaction. Double immunoprecipitation showed that muskelin, p39, and myosin II are components of a single intracellular complex, and suppressing p39 abrogated the interaction between muskelin and the myosin subunits, demonstrating that p39 is required to link muskelin to myosin II. Muskelin is colocalized with myosin regulatory light chain (MRLC) and on stress fibers. The suppression of muskelin reduced Rho-GTP, MRLC phosphorylation, disrupted stress fiber organization, and promoted cell migration, all of which closely mimic the effect of Cdk5 inhibition. Moreover, suppressing muskelin and inhibiting Cdk5 together have no additional effect, indicating that muskelin plays an important role in Cdk5-dependent signaling. p39 is necessary and sufficient for Cdk5-dependent regulation of MRLC phosphorylation, as suppression of p39, but not p35, reduces MRLC phosphorylation. Together, these results demonstrate that p39 specifically links muskelin to myosin II and consequently, to stress fibers and reveal a novel role for muskelin in regulating myosin phosphorylation and cytoskeletal organization.
authors.affiliation: Laboratory of Cellular Oncology, National Cancer Institute, National Institutes of Health, Building 37, Room 4112, Bethesda, MD 20892, USA; Laboratory of Molecular and Developmental Biology, National Eye Institute, National Institutes of Health, Bethesda, MD 20892, USA. Electronic address: tripathib@mail.nih.gov.
authors.forename: Brajendra K
authors.initials: BK
authors.lastname: Tripathi
authors.affiliation: Laboratory of Cellular Oncology, National Cancer Institute, National Institutes of Health, Building 37, Room 4112, Bethesda, MD 20892, USA.
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Lowy
authors.affiliation: Laboratory of Molecular and Developmental Biology, National Eye Institute, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Peggy S
authors.initials: PS
authors.lastname: Zelenka
mesh_terms: D000199:Actins / Q000378:metabolism*; D001665:Binding Sites; D015815:Cell Adhesion Molecules / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D007908:Lens, Crystalline / Q000378:metabolism*; D018994:Myosin Light Chains / Q000378:metabolism; D024462:Myosin Type II / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D022502:Stress Fibers / Q000378:metabolism*; D020741:rho GTP-Binding Proteins / Q000378:metabolism
chemical_list: D000199:Actins; C108298:Cdk5 activator p39; D015815:Cell Adhesion Molecules; D047908:Intracellular Signaling Peptides and Proteins; C503019:MKLN1 protein, human; D018994:Myosin Light Chains; D009419:Nerve Tissue Proteins; D024462:Myosin Type II; D020741:rho GTP-Binding Proteins
keywords: Cdk5; Cell migration; Cytoskeleton; Lens; Muskelin; Myosin; Stress fibers; p39