A new mode of SAM domain mediated oligomerization observed in the CASKIN2 neuronal scaffolding protein
E. Smirnova, J. Kwan, R. Siu, X. Gao, G. Zoidl, B. Demeler, V. Saridakis, L. Donaldson
Cell Commun Signal.,
Analytical ultracentrifugation, Cell signaling, Crystal structure, Neuroscience, Nuclear magnetic resonance, Protein structure, Scaffold protein
CASKIN2 is a homolog of
CASKIN1, a scaffolding protein that participates in a signaling network
with CASK (calcium/calmodulin-dependent serine kinase). Despite a high
level of homology between CASKIN2 and CASKIN1, CASKIN2 cannot bind CASK
due to the absence of a CASK Interaction Domain and consequently, may
have evolved undiscovered structural and functional distinctions.
We demonstrate that the
crystal structure of the Sterile Alpha Motif (SAM) domain tandem
(SAM1-SAM2) oligomer from CASKIN2 is different than CASKIN1, with the
minimal repeating unit being a dimer, rather than a monomer. Analytical
ultracentrifugation sedimentation velocity methods revealed differences
in monomer/dimer equilibria across a range of concentrations and ionic
strengths for the wild type CASKIN2 SAM tandem and a structure-directed
double mutant that could not oligomerize. Further distinguishing CASKIN2
from CASKIN1, EGFP-tagged SAM tandem proteins expressed in Neuro2a
cells produced punctae that were distinct both in shape and size.
This study illustrates a new
way in which neuronal SAM domains can assemble into large macromolecular
assemblies that might concentrate and amplify synaptic responses.
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