The targeting of phospholamban and sarcolipin to the endo / sarcoplasmic reticulum

Abstract

Phospholamban and sarcolipin are two sarcoplasmic reticulum resident C-terminally anchored transmembrane proteins that are involved in the modulation of sarcoplasmic reticulum calcium ATPases (SERCAs).  Very little is known however, about the targeting of these proteins to the sarcoplasmic reticulum and how they are maintained in that cellular location.  To examine the targeting of phospholamban and sarcolipin they were tagged with EGFP so that the trafficking of these proteins within the cell could be observed by fluorescence microscopy.  Using antibodies against known cellular markers it was shown that both phospholamban and sarcolipin exist the endoplasmic reticulum and enter the ERGIC compartment but do not proceed to the trans Golgi.  This indicates that these proteins are maintained in the endoplasmic reticulum mainly by a process of retrieval and not retention.  In addition mutagenesis of the transmembrane domains  of phospholamban and sarcolipin lead to a loss of the retention signal indicating that the transmembrane domains play a major role in the retention process.  Mutagenesis of one of the positively charged residues flanking the transmembrane domain resulted in a inversion of topology of sarcolipin demonstrating that these flanking residues are involved in orientating this tail anchored protein during its insertion into the endoplasmic reticulum.

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