[The Ca(2+)-transport ATPases of smooth muscle]

The cytoplasmic Ca2+ concentration plays a central role in the contraction of smooth muscle cells. The concentration of cytoplasmic Ca2+ is determined for an important part by the operation of Ca(2+)-transport ATPases which extrude Ca2+ from the cell or accumulate the ion in the endoplasmic reticulum. The present work concerns the characterization of the Ca(2+)-transport ATPases of smooth muscle by biochemical, immunochemical and recombinant DNA techniques. This study also includes the investigation of the regulation of the Ca(2+)-transport ATPases and of the expression of associated Ca(2+)-binding proteins. Methods were developed for the purification of endoplasmic reticulum and plasma membranes from smooth-muscle cells. From the study of the phosphorylated transport intermediates and the proteolytic breakdown products, and by using polyclonal and monoclonal antibodies we could conclude that two different Ca(2+)-transport ATPases are expressed in smooth-muscle cells. Of each of these types of Ca(2+)-transport ATPases different isoforms exist. These isoforms were further characterized at the cDNA level and by generating isoform-specific antibodies. One isoform of the plasma-membrane Ca(2+)-pump and two different organellar-type Ca(2+)-pumps have been cloned and sequenced. In smooth-muscle cells, the primary RNA transcripts of the gene of the SERCA2 Ca(2+)-transport ATPase are alternatively processed in three different ways. In neural tissues even a fourth mode of splicing occurs. These different splice modes can be explained by the analysis of the exon/intron structure of the SERCA2 gene. The regulation of the alternative RNA splicing was studied on the stable muscle-cell line BC3H1 during induced myogenic differentiation. From this study we could conclude that the mechanisms responsible for active Ca(2+)-transport in smooth-muscle cells partially resemble those of non-muscle cells, and partially resemble the corresponding system in cardiac cells, but not those in skeletal muscle. A similar conclusion was reached concerning the regulation of the Ca(2+)-transport ATPase of the endoplasmic reticulum via the phosphorylation of phospholamban, and for the expression of the Ca(2+)-binding protein calsequestrin.(ABSTRACT TRUNCATED AT 400 WORDS)