There is an absence or deficiency of N-acetylglucosamine phosphotrans-
ferase and an absence of mannose-6-phosphate (M6P) on the lysosomal
enzymes. The failure to add M6P in the cis-Golgi results in inappropriate
vesicular segregation by M6P receptors in the trans–Golgi network (TGN).
The default pathway is transport to the cell membrane and secretion from
the cell by exocytosis for proteins lacking M6P. Lysosomal enzymes are
secreted into the bloodstream, and undigested substrates build up within
the cells. There is no missorting back to the Golgi (answer a).Peroxisomal
enzymes, which are sorted by the presence of three specific amino acids
located at the C-terminus: Ser–Lys–Leu–COO−, are not affected (answer
b).KDEL(answer c)is the signal used for retrieval of proteins from the
Golgi back to the endoplasmic reticulum. SNAREs [soluble-N-ethylemalemide
sensitive factor (NSF) attachment protein receptors] are the receptors for
SNAPs [soluble-N-ethylemalemide sensitive factor (NSF) attachment pro-
teins] and bind vesicles to membranes (answer d).Trafficking to other
structures, such as the nucleus and mitochondria, is regulated by nuclear
localization signals (NLSs) or an N-terminal signal peptide, respectively.
Cell Biology: Intracellular Trafficking Answers 137