Unbiased Thiol-Labeling and Top-Down Proteomic Analyses Implicate Multiple Proteins in the Late Steps of Regulated Secretion

Kendra L Furber; Peter S Backlund; Alfred L Yergey; Jens R Coorssen

doi:10.3390/proteomes7040034

Unbiased Thiol-Labeling and Top-Down Proteomic Analyses Implicate Multiple Proteins in the Late Steps of Regulated Secretion

Proteomes. 2019 Sep 27;7(4):34. doi: 10.3390/proteomes7040034.

Authors

Kendra L Furber¹, Peter S Backlund², Alfred L Yergey³, Jens R Coorssen⁴

Affiliations

¹ Northern Medical Program, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada. kendra.furber@unbc.ca.
² Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA. pbacklund@verizon.net.
³ Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA. alyergey@gmail.com.
⁴ Department of Health Sciences, Faculty of Applied Health Sciences and Department of Biological Sciences, Faculty of Mathematics & Science, Brock University, St. Catharines, ON L2S 3A1, Canada. jcoorssen@brocku.ca.

Abstract

Regulated exocytosis enables temporal and spatial control over the secretion of biologically active compounds; however, the mechanism by which Ca²⁺ modulates different stages of exocytosis is still poorly understood. For an unbiased, top-down proteomic approach, select thiol- reactive reagents were used to investigate this process in release-ready native secretory vesicles. We previously characterized a biphasic effect of these reagents on Ca²⁺-triggered exocytosis: low doses potentiated Ca²⁺ sensitivity, whereas high doses inhibited Ca²⁺ sensitivity and extent of vesicle fusion. Capitalizing on this novel potentiating effect, we have now identified fluorescent thiol- reactive reagents producing the same effects: Lucifer yellow iodoacetamide, monobromobimane, and dibromobimane. Top-down proteomic analyses of fluorescently labeled proteins from total and cholesterol-enriched vesicle membrane fractions using two-dimensional gel electrophoresis coupled with mass spectrometry identified several candidate targets, some of which have been previously linked to the late steps of regulated exocytosis and some of which are novel. Initial validation studies indicate that Rab proteins are involved in the modulation of Ca²⁺ sensitivity, and thus the efficiency of membrane fusion, which may, in part, be linked to their previously identified upstream roles in vesicle docking.

Keywords: 2-dimensional gel electrophoresis; Rab GTPase; calcium; exocytosis; secretory vesicle; thiol-reactivity.