The human
body comprises a great many cells, each of which contains many subcellular
units known as organelles (e.g., nucleus, the Golgi apparatus, and endoplasmic
reticulum). Signals are exchanged
frequently between cells and between organelles through “membrane
traffic”, in which membrane-wrapped substances
are transported. However, much
remains unknown about this process.
Our laboratory focuses on the secretory phenomena (i.e., transport of
secretory vesicles in neurons and endocrine cells) and on the melanin transport
in melanocytes and tries to identify “key molecules”
responsible for the secretion events and melanin transport. The aim of our research is to elucidate
the molecular mechanism of membrane traffic by use of molecular biology, cell
biology, biochemistry, and molecular imaging techniques.
We
previously showed that an abundant synaptic vesicle protein synaptotagmin I (Syt I) regulates synaptic vesicle
exocytosis (i.e., neurotransmitter release) and endocytosis in neurons. Syt I consists of a single N-terminal
transmembrane domain and C-terminal tandem C2 calcium/phospholipid-binding
domains (named C2A domain and C2B domain).
These two C2 domains are functional domains of Syt I, because
functionally blocking antibody against the C2A domain (or the C2B domain)
inhibited synaptic vesicle fusion step (or synaptic vesicle recycling step) (PNAS, 2000; PNAS, 2004).
We have
identified novel synaptotagmin-related molecules that contain tandem C2 domains
at the C terminus (named Slp, synaptotagmin-like
protein) and their related protein Slac2
(Slp homologue lacking C2 domains).
Both Slp and Slac2 contain the conserved domain (named SHD, Slp homology
domain) at their N terminus, and we found that the SHD functions as an effector
domain for small GTPase Rab27A, which is specifically present on melanosomes in
mammalian skin melanocytes (JBC,
2002). We further found that two
Rab27A effectors, Slac2-a (also called melanophilin) and Slp2-a, are abundantly
expressed on melanosomes and sequentially regulate melanosome transport in
melanocytes (Nature Cell Biol.,
2004). Slac2-a simultaneously
interacts with Rab27A on the melanosome and with an actin-based motor myosin
Va, and the resultant tripartite protein complex (Rab27A-Slac2-a-myosin Va)
mediates actin-based melanosome transport (MCB,
2003). After actin-dependent
melanosome transport, the second Rab27A effector Slp2-a promotes the anchoring
of melanosomes to the plasma membrane of melanocytes through direct interaction
of the C2A domain with phosphatidylserine (PS). Moreover, we found that some other
Rab27A effectors (e.g., Slp4-a and rabphilin) promote docking of Rab27-bound
organelles (e.g., secretory granules) to the plasma membrane (JBC, 2005; MBC, 2006; JCS, 2006), the
same as the role of Slp2-a in anchoring of melanosomes to the to the plasma
membrane.
Currently,
we are focusing on the function of “Rab small
GTPases”, key regulators in membrane traffic in
all eukaryotes. Approximately 60
Rab members are present in mammals and they are thought to be involved in
specific step or specific type of membrane traffic. However, because of their large numbers
the precise function of most mammalian Rab isoforms is still unknown. We recently developed new tools for
comprehensive analysis of the mammalian Rab family (named “Rab
panels”), and their development has paved the
way for systematic investigation of the involvement of mammalian Rab isoforms
in specialized membrane trafficking events (MCP, 2008; Traffic, 2010; Traffic, 2011), including melanosome
biogenesis/transport (MBC, 2009; JBC, 2011), autophagy (MBC, 2008; JCB, 2011), and neuronal polarity
formation (JBC, 2012).