PEF1 (penta-EF-hand domain containing 1) is a calcium-binding protein that functions as a calcium-dependent adapter in protein complex assembly and ER-Golgi transport. Primary Function: PEF1 acts as an adapter that bridges unrelated proteins or stabilizes weak protein-protein complexes in response to calcium increases 1. Mechanism: In response to cytosolic calcium elevation, PEF1 forms a heterodimer with PDCD6 (ALG2) that interacts with the BCR(KLHL12)-SEC31 complex, promoting monoubiquitination of SEC31 and subsequent collagen export 1. This calcium-dependent co-adaptor initiates CRL3 complex formation by releasing KLHL12 from assembly inhibitors at the endoplasmic reticulum, coupling target recognition to ubiquitylation 2. However, evidence suggests PEF1 may also dissociate from PDCD6 in the presence of calcium and act as a negative regulator of ER-Golgi transport 3. Disease Relevance: PEF1's calcium-dependent regulation of CUL3 ubiquitin ligase activity is essential for neural crest specification; aberrant CUL3 activation has been linked to autism, schizophrenia, and hypertension 1. Clinical Significance: PEF1 was identified as upregulated in TRAIL-sensitive myeloma cells, suggesting potential involvement in apoptosis-related pathways relevant to cancer therapeutics 4. The protein's role in collagen secretion and craniofacial bone formation highlights developmental importance.