Mammalian Ykt6 Is a Neuronal SNARE Targeted to a Specialized Compartment by its Profilin-like Amino Terminal Domain

Abstract

SNAREs are required for specific membrane fusion throughout the endomembrane system. Here we report the characterization of rat ykt6, a prenylated SNARE selectively expressed in brain neurons. Immunofluorescence microscopy in neuronal and neuroendocrine cell lines revealed that membrane-associated ykt6 did not colocalize significantly with any conventional markers of endosomes, lysosomes, or the secretory pathway. However, ykt6-containing membranes displayed very minor overlaps with lysosomes and dense-core secretory granules and were similar to lysosomes in buoyant density. Thus, ykt6 appears to be specialized for the trafficking of a unique membrane compartment, perhaps related to lysosomes, involved in aspects of neuronal function. Targeting of this SNARE to the ykt6 compartment was mediated by its profilin-like amino-terminal domain, even in the absence of protein prenylation. Although several other R-SNAREs contain related amino-terminal domains, only the ykt6 version was able to confer the specialized localization. Rat ykt6, which contains an arginine in its SNARE motif zero-layer, was found to behave like other R-SNAREs in its SNARE assembly properties. Interestingly, cytosolic ykt6, constituting more than half of the total cellular pool, appeared to be conformationally inactive for SNARE complex assembly, perhaps indicative of a regulatory mechanism that prevents promiscuous and potentially deleterious SNARE interactions.

The Identification of a Novel Endoplasmic Reticulum to Golgi SNARE Complex Used by the Prechylomicron Transport Vesicle

“…In the other ERGolgi SNARE complex that has been described, also in NRK cells, Ykt6 was the v-SNARE, with syntaxin 5, GOS28, and rBet1 as the t-SNAREs (30). An extensive tissue survey in rats has shown that Ykt6 is predominantly found in brain in a lysosome-related compartment (31). rBet1, another potential SNARE protein, is predominantly an ER-intermediate compartment resident protein recycling from the Golgi (32) and is present in intestinal ER and PCTV (4).…”

Vesicle-associated membrane protein 7 is expressed in intestinal ER

“…VAMP7, an additional member of the longin family (TI-VAMP, SYBL1) (Filippini et al, 2001), has not been described as an ER-related SNARE, but has been shown to be predominately associated with endosomes (Advani et al, 1999) in mammals. VAMP7 (Advani et al, 1999) and Sec22b have both been shown to have a widespread tissue distribution whereas Ykt6 is predominantly found in brain in a lysosome-related compartment (Hasegawa et al, 2003).…”

Intramolecular protein-protein and protein-lipid interactions control the conformation and subcellular targeting of neuronal Ykt6

“…For example, rat Ykt6 is highly enriched in brain (Hasegawa et al, 2003). This finding was surprising, because a SNARE involved in constitutive biosynthetic transport to the lysosome (as yeast Ykt6p is) would probably be very widely expressed.…”