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Abstract
During Dictyostelium development, prespore cells secrete acyl-CoA binding protein
(AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation
factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have
found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached
peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and,
thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not
secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell
surface). GRASP is not required for conventional protein secretion, growth, and the
viability of vegetative cells. Our findings reveal a physiological role of GRASP and
provide a means to understand unconventional secretion and its role in development.