Jiliu Xu

Identification of a Candidate Human Spectrin Src Homology 3 Domain-binding Protein Suggests a General Mechanism of Association of Tyrosine Kinases with the Spectrin-based Membrane Skeleton

Spectrin is a widely expressed protein with specific isoforms found in erythroid and nonerythroid cells. Spectrin contains an Src homology 3 (SH3) domain of unknown function. A cDNA encoding a candidate spectrin SH3 domain-binding protein was identified by interaction screening of a human brain expression library using the human erythroid spectrin (αI) SH3 domain as a bait. Five isoforms of the αI SH3 domain-binding protein mRNA were identified in human brain. Mapping of SH3 binding regions revealed the presence of two αI SH3 domain binding regions and one Abl-SH3 domain binding region. The gene encoding the candidate spectrin SH3 domain-binding protein has been located to human chromosome 10p11.2 → p12. The gene belongs to a recently identified family of tyrosine kinase-binding proteins, and one of its isoforms is identical to e3B1, an eps8-binding protein (Biesova, Z., Piccoli, C., and Wong, W. T. (1997)Oncogene 14, 233–241). Overexpression of the green fluorescent protein fusion of the SH3 domain-binding protein in NIH3T3 cells resulted in cytoplasmic punctate fluorescence characteristic of the reticulovesicular system. This fluorescence pattern was similar to that obtained with the anti-human erythroid spectrin αIΣI/βIΣI antibody in untransfected NIH3T3 cells; in addition, the anti-αIΣI/βIΣI antibody also stained Golgi apparatus. Immunofluorescence obtained using antibodies against αIΣI/βIΣI spectrin and Abl tyrosine kinase but not against αII/βII spectrin colocalized with the overexpressed green fluorescent protein-SH3-binding protein. Based on the conservation of the spectrin SH3 binding site within members of this protein family and published interactions, a general mechanism of interactions of tyrosine kinases with the spectrin-based membrane skeleton is proposed.

Inhibitors of protein phosphatase-2A from human brain structures, immunocytological localization and activities towards dephosphorylation of the Alzheimer type hyperphosphorylated tau

Protein phosphatase (PP)‐2A, which regulates the phosphorylation of tau, is regulated by two endogenous inhibitor proteins, I 1 PP 2 A and I 2 PP 2 A , in mammalian tissues. Here, we report the cloning of I 1 PP 2 A and I 2 PP 2 A from human brain, and show that in PC12 cells and in I 1 PP 2 A ‐ GFP or I 2 PP 2 A ‐ GFP transfected NIH3T3 and human neural progenitor cells, I 1 PP 2 A is localized mostly in the cell cytoplasm and I 2 PP 2 A mostly in the nucleus. The recombinant I 1 PP ‐ 2 A and I 2 PP ‐ 2 A inhibit PP‐2A activity towards hyperphosphorylated tau in vitro; the dephosphorylation of the hyperphosphorylated tau at specific sites is selectively inhibited. Overexpression of I 1 PP 2 A as well as I 2 PP 2 A results in tau hyperphosphorylation and degeneration of PC 12 cells.

Altered binding of mutated presenilin with cytoskeleton-interacting proteins1

The majority of familial Alzheimers disease (AD) cases are linked to mutations on presenilin 1 and 2 genes (PS1 and PS2). The normal function of the proteins and the mechanisms underlying early‐onset AD are currently unknown. To address this, we screened an expression library for proteins that bind differentially to the wild‐type PS1 and mutant in the large cytoplasmic loop (PS1L). Thus we isolated the C‐terminal tail of the 170 kDa cytoplasmic linker protein (CLIP‐170) and Reed–Sternberg cells of Hodgkins disease‐expressed intermediate filament‐associated protein (Restin), cytoplasmic proteins linking vesicles to the cytoskeleton. PS1L binding to CLIP‐170/restin requires Ca2+. Treating cells with thapsigargin or ionomycin increased the mutated PS1 in CLIP‐170 immunoprecipitates. Further, PS1 and CLIP‐170 co‐localize in transfected cells and neuronal cultures.

Monoclonal antibodies to αI spectrin Src homology 3 domain associate with macropinocytic vesicles in nonerythroid cells

Spectrins represent a family of membrane-associated proteins responsible for membrane flexibility and cell shape in erythrocytes, and probably in most nonerythroid cells. Spectrin functions as a tetramer consisting of two heterodimers each containing two subunits termed alpha and beta. In humans, alphaI and alphaII spectrins but not beta spectrins are characterized by the presence of an Src homology 3 (SH3) domain. As a tool to investigate the function of spectrin SH3 domains we derived several monoclonal antibodies (mAb) to the recombinant human alphaI or alphaII spectrin SH3 domain. Immunostaining using these monoclonal antibodies indicated expression of alphaI spectrin in cell bodies and alphaII spectrin in neurites of granule neurons in mouse primary cerebellar cultures. Monoclonal antibodies reactive to alphaI spectrin SH3 domain indicated expression of a protein(s) containing an alphaI-like SH3 domain in cytoplasmic vesicular-like structures in GFAP-positive cells in these cultures. In NIH 3T3 fibroblasts, these antibodies label macropinocytic vesicles. Together, these data and Western blotting results suggest expression of at least three spectrin-SH3 domain antibody-reactive proteins.