Fast inactivation of Shal (K
v
4) K
+
channels is regulated by the novel interactor
SKIP3 in Drosophila neurons
Fengqiu Diao, Girma Waro, Susan Tsunoda
⁎
Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA
abstractarticle info
Article history:
Received 21 January 2009
Revised 8 May 2009
Accepted 13 May 2009
Available online 20 May 2009
Shal K
+
(K
v
4) channels across species carry the major A-type K
+
current present in neurons. Shal currents are
activated by small EPSPs and modulate post-synaptic potentials, backpropagation of action potentials, and
induction of LTP. Fast inactivation of Shal channels regulates the impact of this post-synaptic modulation. Here,
we introduce SKIP3, as the first protein interactor of Drosophila Shal K
+
channels. The SKIP gene encodes three
isoforms with multiple protein–protein interaction domains. SKIP3 is nervous system specific and co-localizes
with Shal channels in neuronal cell bodies, and in puncta along processes. Using a genetic deficiency of SKIP,
we show that the proportion of neurons displaying a very fast inactivation, consistent with Shal channels
exclusively in a “fast” gating mode, is increased in the absence of SKIP3. As a scaffold-like protein, SKIP3 is
likely to lead to the identification of a novel regulatory complex that modulates Shal channel inactivation.
© 2009 Elsevier Inc. All rights reserved.
Introduction
Shal (K
v
4) is the most highly conserved of the voltage-gated K
+
channel gene subfamilies, sharing 82% amino acid identity between
Drosophila and mouse (Pak et al., 1991) and generating a transient
A-type K
+
current from C. elegans to humans (Fawcett et al., 2006;
Isbrandt et al., 2000; Jerng et al., 2004; Salkoff et al., 1992). Molecular,
genetic, and pharmacological studies have identified Shal channels as
underlying the somato-dendritic A-type K
+
current in most neurons
(Jerng et al., 2004). With a hyperpolarized voltage operating range
typical of A-type currents, K
v
4.2 currents in hippocampal neurons
have been shown to act at subthreshold potentials, regulating the
integration of high-frequency trains of synaptic input (Ramakers and
Storm, 2002), the shape of mEPSCs (Kim et al., 2007), backpropagat-
ing action potentials in dendrites (Cai et al., 2004; Chen et al., 2006;
Kim et al., 2005), and induction of long-term potentiation (LTP)
(Chen et al., 2006; Kim et al., 2007). Rapid inactivation of Shal
currents directly affects the duration of its effect on membrane
potential, and therefore plays a critical role in Shal channel
modulation of post-synaptic potentials.
Regulation of inactivation rates then is likely to be an important
mechanism for modulating neuronal firing frequency and integration
of post-synaptic potentials. Inactivation of mammalian K
v
4 channels
has been shown to be regulated by auxiliary subunits, including K
+
channel interacting proteins (KChIPs) (An et al., 2000) and dipeptidyl
aminopeptidase-like proteins (DPPX) (Nadal et al., 2003). In Droso-
phila, there is only one Shal K
+
channel gene, and the encoded protein
underlies the predominant transient A-type current present in
virtually all neurons (Tsunoda and Salkoff, 1995a). Interestingly, the
inactivation rate of these currents varies over several orders of
magnitude (Tsunoda and Salkoff, 1995a). Single Shal channels in
Drosophila have been shown to adopt either a “fast” or “slow” gating
mode, giving rise to whole-cell currents with different rates of
inactivation (Tsunoda and Salkoff, 1995a). These Shal channels have
been proposed to switch between the two gating modes (Tsunoda and
Salkoff, 1995a). The molecular mechanisms regulating gating mode
changes, or proportions of channels in each gating mode, however, are
still unknown. Thus, Drosophila Shal K
+
channels present an
interesting model system for studying additional mechanisms under-
lying the regulation of K
+
current inactivation.
In this study, we identify a novel protein, SKIP3 (Shal K
+
Channel
Interacting Protein-3), as the first interactor of Drosophila Shal K
+
channels. We show that SKIP isoforms are expressed specifically in the
nervous system, and that SKIP3 is likely the only isoform that interacts
with Shal channels. To examine the function of SKIP3 in neurons, we
identify a genetic deficiency of SKIP (Df(3R)Exel6190). We show that a
higher percentage of homozygous Df(3R)Exel6190 neurons display
Shal currents with inactivation rates that correspond to Shal channels
exclusively in the “fast” gating mode. Altogether, our study introduces
SKIP3 as a novel Shal K
+
channel interactor that regulates the
inactivation of Shal K
+
channels.
Results
Identification of a novel Shal K
+
channel interactor, SKIP3
Little is known about the mechanism(s) underlying the variable
inactivation rate of Drosophila Shal K
+
channels. We hypothesized that
protein interactor(s) might function in the regulation of Shal channel
Molecular and Cellular Neuroscience 42 (2009) 33–44
⁎ Corresponding author. Fax: +1 617 353 8484.
E-mail address: tsunoda@bu.edu (S. Tsunoda).
1044-7431/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.mcn.2009.05.003
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