Tyrosine-phosphorylated caveolin-1: immunolocalization and molecular characterization

Concentration of caveolin-1 in the cleavage furrow as revealed by time-lapse analysis

Caveolin-1 is a major component of caveolae. Recent studies have suggested a possible role of caveolin-1 in cell transformation and normal cell proliferation. To observe the behavior of caveolin-1 in living mitotic cells, we prepared cDNA constructs encoding the chimeric protein of alpha- or beta-caveolin-1 and green fluorescent protein (GFP) and transfected culture cells with them. Correct targeting of the chimera to the caveolae was confirmed by colocalization with the caveolar inositol 1,4,5-trisphosphate receptor-like protein. By time-lapse observation of mitotic MDCKII cells, the GFP-caveolin-1 chimeras were seen throughout the plasma membrane before cell division, but became markedly concentrated at the cleavage furrow during cytokinesis. Accumulation around the spindle poles was also observed at late telophase. The result showed that caveolin-1 undergoes a drastic distributional change during cell division and suggested that the protein may be involved in the cytokinetic process.

Caveolin-1 isoforms are encoded by distinct mRNAs. Identification Of mouse caveolin-1 mRNA variants caused by alternative transcription initiation and splicing

By searching the EST database with the known cDNA sequence encoding alpha-caveolin-1 (full-length: FL), we found a variant having a hitherto unknown sequence in place of the first exon (5'-end variant: 5'V). The expression level of 5'V mRNA was equivalent to that of FL mRNA. The entire sequences of FL and 5'V mRNA were determined by 3'- and 5'-RACE analysis; their sizes were 2484 bp and 2533 bp, respectively, and the sequences were identical except for the region of the first exon. By Northern blotting, FL and 5'V mRNAs showed the same tissue distribution, and were intensely expressed in the lung, heart, and skeletal muscle. Analyzing the protein production from these mRNAs using green fluorescent protein as a tag, we found FL mRNA to produce the alpha-isoform predominantly, but to form little beta-isoform. The production of the beta-isoform from 5'V mRNA was also demonstrated. By sequence analysis of the first intron of the caveolin-1 gene, a TATA box was found at 28 bp upstream of the transcription initiation site for 5'V mRNA. This is the first demonstration of caveolin-1 mRNA variants generated by alternative transcription initiation, and it indicates that the two isoforms of caveolin-1 are produced from two distinct mRNAs.

Tyrosine phosphorylation of caveolin-1 in the endothelium

Caveolin-1, a scaffolding protein of caveolae, is known to be tyrosine-phosphorylated by Src kinases. Recently we generated a specific antibody to caveolin-1 phosphorylated at tyrosine-14 (PY14) (R. Nomura and T. Fujimoto, 1999, Mol. Biol. Cell 10, 975-986). In the present study, by applying PY14 to sections of normal rat tissues, we found that tyrosine phosphorylation of caveolin-1 occurred in limited locations, including the endothelium of the continuous capillaries and small venules. Cultured endothelial cells were not labeled by PY14 under a standard culture condition, but became positively labeled when exposed to oxidative stresses and/or tyrosine phosphatase inhibitors. The reaction was prohibited by pretreating the cells with herbimycin A or genistein. Vasoactive reagents or physical stimuli did not cause the phosphorylation. Concomitant with the tyrosine phosphorylation, the number of invaginated caveolae decreased drastically, and vesicles labeled intensely for caveolin-1 appeared in the cytoplasm; the average diameter of the vesicles was larger than that of caveolae. The result implies that tyrosine phosphorylation of caveolin-1 occurs at tyrosine-14 in the normal rat endothelium in vivo and may induce caveolar vesiculation and/or fusion.