Direct interaction of the 170 kDa isoform of synaptojanin 1 with clathrin and with the clathrin adaptor AP-2

Abstract

Synaptojanin 1, a polyphosphoinositide phosphatase, is expressed as two major alternatively spliced isoforms of 145 kDa (SJ145) and 170 kDa (SJ170), which are thought to have pleiotropic roles in endocytosis, signaling and actin function. SJl45 is highly enriched in nerve terminals where it participates in clathrin-dependent synaptic vesicle recycling. SJ170, which differs from SJ145 by the presence of a carboxy-terminal extension, is the predominant isoform in developing neurons and is expressed in a variety of tissues. The carboxy-terminal domain unique to SJ170 was previously shown to bind Eps15, a protein involved in receptor-mediated endocytosis. Here, we show that the same domain also binds clathrin and the clathrin adaptor AP-2. These interactions occur both in vitro and in vivo and are direct. Binding of AP-2 is mediated by the ear domain of its α-adaptin subunit and binding of clathrin by the amino-terminal domain of its heavy chain. Overexpression in chinese hamster ovary (CHO) cells of full-length SJ170 or its unique carboxyterminal region caused mislocalization of Eps15, AP-2 and clathrin, as well as inhibition of clathrin-dependent transferrin uptake. These findings suggest a close association of SJ170 with the clathrin coat and provide new evidence for its physiological role in the regulation of clathrin coat dynamics.