ABSTRACT
We describe here a novel sarcomeric 145-kD protein, myopalladin, which tethers together the COOH-terminal Src homology 3 domains of nebulin and nebulette with the EF hand motifs of alpha-actinin in vertebrate Z-lines. Myopalladin's nebulin/nebulette and alpha-actinin-binding sites are contained in two distinct regions within its COOH-terminal 90-kD domain. Both sites are highly homologous with those found in palladin, a protein described recently required for actin cytoskeletal assembly (Parast, M.M., and C.A. Otey. 2000. J. Cell Biol. 150:643-656). This suggests that palladin and myopalladin may have conserved roles in stress fiber and Z-line assembly. The NH(2)-terminal region of myopalladin specifically binds to the cardiac ankyrin repeat protein (CARP), a nuclear protein involved in control of muscle gene expression. Immunofluorescence and immunoelectron microscopy studies revealed that myopalladin also colocalized with CARP in the central I-band of striated muscle sarcomeres. Overexpression of myopalladin's NH(2)-terminal CARP-binding region in live cardiac myocytes resulted in severe disruption of all sarcomeric components studied, suggesting that the myopalladin-CARP complex in the central I-band may have an important regulatory role in maintaining sarcomeric integrity. Our data also suggest that myopalladin may link regulatory mechanisms involved in Z-line structure (via alpha-actinin and nebulin/nebulette) to those involved in muscle gene expression (via CARP).
Subject(s)
Muscle Proteins/metabolism , Muscle, Skeletal/physiology , Sarcomeres/metabolism , Actinin/metabolism , Amino Acid Sequence , Animals , Blotting, Northern , Cells, Cultured , EF Hand Motifs/genetics , Humans , Mice , Microscopy, Fluorescence , Molecular Sequence Data , Muscle Proteins/chemistry , Muscle Proteins/genetics , Muscle, Skeletal/cytology , Myocardium/cytology , Nuclear Proteins/metabolism , Phylogeny , Protein Binding , Protein Structure, Tertiary , Rabbits , Repressor Proteins/metabolism , Sarcomeres/ultrastructure , Sequence Alignment , Two-Hybrid System TechniquesABSTRACT
Approximately 20% of B-lineage acute lymphoblastic leukemias are not cured by traditional chemotherapy. The possibility was examined that residual leukemic cells that potentially contribute to relapse are harbored in association with fibroblastic stromal cells in the bone marrow. Modulation of cytarabine (Ara-C) and etoposide (VP-16) efficacy by bone marrow stromal cells in vitro was investigated. Stromal cell coculture was shown to sustain the proliferation of B-lineage leukemic cells and to reduce leukemic cell apoptosis when exposed to Ara-C or VP-16. Direct contact with stromal cells was essential for the protection of leukemic cells during chemotherapy, whereas soluble factors had negligible effect. Specifically, signaling mediated through interaction with the stromal cell adhesion molecule VCAM-1 was required to maintain the maximum viability of leukemic cells during Ara-C and VP-16 exposure. In contrast, the interaction of leukemic cells with fibronectin did not confer significant resistance to either chemotherapeutic agent. These observations suggest a role for the bone marrow microenvironment in modulating the response of B-lineage leukemic cells to Ara-C or VP-16, and they indicate specific molecular interactions that may be important in determining the sensitivity of leukemic cells to treatment. (Blood. 2000;96:1926-1932)