Nucleobindin 1 is a Calcium Regulated Guanine Nucleotide Dissociation Inhibitor of GαI1

Abstract

Nucleobindin 1 (NUCB1) is a widely expressed multi-domain calcium-binding protein whose precise physiological and biochemical functions are not well understood. We engineered and heterologously expressed a soluble form of NUCB1 (sNUCB1) and show that sNUCB1 exists as a Calcium binding dimer in solution and binding to Calcium causes conformational changes in sNUCB1. Earlier reports suggested that NUCB1 might interact with heterotrimeric G protein α subunits. We show that dimeric sNUCB1 binds to Gαi1 and that calcium-binding inhibits the interaction. The binding of sNUCB1 to Gαi1 inhibits its basal rate of GDP release and slows its rate and extent of GTPγS uptake. Additionally, our tissue culture experiments show that sNUCB1 prevents receptor-mediated Gαi-dependent inhibition of adenylyl cyclase (AC). Thus, we conclude that sNUCB1 is a calcium dependent guanine-nucleotide dissociation inhibitor (GDI) for Gαi1. To our knowledge sNUCB1 is the first example of a calcium-dependent GDI for heterotrimeric G proteins. We also show that the mechanism of GDI activity of sNUCB1 is unique and does not arise from the consensus Goloco motif found in RGS proteins. We propose that cytoplasmic NUCB1 might function to regulate heterotrimeric G protein trafficking and signalling.