Major calcium-binding protein of the Golgi which may have a role in calcium homeostasis (PubMed:7890746). Acts as a non-receptor guanine nucleotide exchange factor which binds to and activates alpha subunits of guanine nucleotide-binding proteins (G proteins) (PubMed:21653697). Interacts (via GBA motif) with guanine nucleotide-binding protein G(i) alpha subunits GNAI1, GNAI2 and GNAI3 with higher affinity for GNAI1 and GNAI3 than for GNAI2 (PubMed:10639138, PubMed:21653697). Preferentially interacts with inactive rather than active GNAI3 (PubMed:21653697). Interaction with GNAI3 is inhibited when NUCB1 binds calcium, probably due to a conformational change which renders the GBA motif inaccessible (PubMed:21653697). A small fraction of the protein may be cytoplasmic. In bone at least part of it is found in the extracellular matrix and in the culture medium of calvaria explants. Minor constituent of the mineralized matrix of bone. Detected in calvaria, rib cartilage, liver, kidney, spleen, brain, lung, skeletal and heart muscle with highest expression in calvaria and approximately half the amount in kidney, liver and brain. The EF-hand domains are unfolded in the absence of Ca(2+) and fold upon Ca(2+) addition. The GBA (G-alpha binding and activating) motif mediates binding to the alpha subunits of guanine nucleotide-binding proteins (G proteins). Discovered as DNA-binding protein in the serum of lupus-prone mice. Belongs to the nucleobindin family.