Structural diversity of the voltage-dependent Ca2+ channel α1E-subunit

Voltage-operated Ca2+ channels are heteromultimeric proteins. Their structural diversity is caused by several genes encoding homologous subunits and by alternative splicing of single transcripts. Isoforms of al subunits, which contain the ion conducting pore, have been deduced from each of the six cDNA sequences cloned so far from different species. The isoforms predicted for the alpha(1E) subunit are structurally related to the primary sequence of the amino terminus, the centre of the subunit (II-III loop), and the carboxy terminus. Mouse and human alpha(1E) transcripts have been analysed by reverse transcription-polymerase chain reaction and by sequencing of amplified fragments. For the II-III loop three different alpha(1E) cDNA fragments are amplified from mouse and human brain, showing that isoforms originally predicted from sequence alignment of different species are expressed in a single one. Both predicted alpha(1E) cDNA fragments of the carboxy terminus are identified in vivo, Two different alpha(1E) constructs, referring to the major structural difference in the carboxy terminus, were stably transfected in HEK293 cells. The biophysical properties of these cells were compared in order to evaluate the importance in vitro of the carboxy terminal insertion found in vivo. The wild-type alpha(1E) subunit showed properties, typical for a high-voltage activated Ca2+ channel. The deletion of 43 amino acid residues at the carboxy terminus does not cause significant differences in the current density and the basic biophysical properties. However, a functional difference is suggested, as in embryonic stem cells, differentiated in vitro to neuronal cells, the pattern of transcripts indicative for different alpha(1E) isoforms changes during development. In human cerebellum the longer alpha(1E) isoform is expressed predominantly. Although, it has not been possible to assign functional differences to the two alpha(1E) constructs tested in vitro, the expression pattern of the structurally related isoforms may have functional importance in vivo.