Mass spectrometric characterization of gelsolin isoforms

Gelsolin is the most widely expressed member of the actin capping and severing family of proteins. There are two isoforms of gelsolin: isoform 1, a secretory (plasma) protein that is 51 amino acids longer than isoform 2, a cytosolic protein, at the N-terminus; the first 27 amino acids is a signal sequence. Both isoforms are coded by a single gene and differ as a result of alternative initiation site/splicing. The level of gelsolin in the blood and cerebrospinal fluid (CSF) is altered in many diseases including amyloidoses and other neurodegenerative disorders. Although quantitative analysis of gelsolin has been reported, lack of suitable antibodies makes it impossible to differentiate these two isoforms by immunodetection techniques and no other technique is available. Therefore, ambiguity exists whether gelsolin present in circulation is isoform 1 or also isoform 2 released from lysed cells. We report in this communication a mass spectrometric approach to identify isoform 1 of gelsolin immunopurified from human plasma and CSF. Recombinant isoform 1 was used as reference.