GRASP65 gets the chop

Early in apoptosis, caspase 3 chops up the Golgi stacking protein GRASP65 to fragment the Golgi, according to results presented by Lane et al. on page 495. The fragmentation may cut the Golgi into manageable pieces, and allow dispersion of the pieces so that they can be incorporated into apoptotic bodies near the disintegrating plasma membrane.

Lane et al. map the relevant cleavage sites on GRASP65 to the COOH terminus of the protein. A GRASP65 protein with these sites mutated is no longer cleaved, and the mutant protein preserves ∼1/4 of the Golgi stacking after apoptosis induction. Given that GRASP65 is located only in the cis Golgi, it might be expected to protect at most half of the Golgi from a loss of stacking. Lane et al. suspect that other caspase substrates control degradation of the rest of the Golgi, although they show that a number of other known Golgi stacking proteins are not cleaved during apoptosis.The results with mutant GRASP65 underscore the in vivo importance of GRASP65 in construction of the Golgi, which was previously only hinted at in in vitro assays. This function may be performed by the cleavable COOH terminus of the protein, whereas the vesicle tethering function of GRASP65 relies on the NH₂ terminus. The mechanisms by which the COOH terminus carries out its job, including any protein partners, remain unknown. ▪