The tethering protein p115 is essential for ER-Golgi traffic. Immunofluorescence and EM studies localize p115 to membranes of ERES, ERGIC

and the Golgi. We have used GFP-tagged p115 and fluorescence recovery after photobleaching (FRAP) to explore p115 interaction with

membranes in live cells. FRAP of p115 within the Golgi is rapid, with a mean t1/2 ~11 sec. The mobility is independent of p115 interactions with

GM130 and giantin because the FRAP of a p115 mutant missing the GM130/giantin binding site is analogous (t1/2 ~13 sec) to that of full-length

p115. p115 has been shown to directly bind SNAREs in vitro. We explored the role of SNAREs in p115 dynamics in vivo. We prevented the

disassembly of SNARE complexes by expressing an inactive (E329Q) mutant of NSF. In transfected cells, the FRAP of p115 is increased to t1/2

~21 sec, nearly twice that of control cells. To more rigorously inhibit SNARE complex disassembly, we perturbed NSF function by treating cells

with N-ethylmaleimide (NEM). In NEM-treated cells, FRAP of p115 is significantly delayed (t1/2 >5 min). The findings suggest that p115

mobility is directly linked to the cycle of SNARE complex formation and disassembly. We also explored the dynamics of p115 in cells

expressing a dominant negative N121I mutant of Rab1. N121I inhibits ER-Golgi traffic, presumably by preventing activation of endogenous

Rab1. In N121I-expressing cells, FRAP of p115 is significantly faster, t1/2 ~4 sec. Our findings support a model in which both SNAREs and

Rab1 regulate p115 integration into a stabilized membrane complex. The full complement of this complex is unknown, and p115 may act on

member(s) of this complex to facilitate traffic.