Properties and proximity proteomics of synaptopodin provide insight into the molecular organization of the spine apparatus of dendritic spines

The spine apparatus is a specialization of the neuronal ER in dendritic spines consisting of stacks of interconnected cisterns separated by a dense matrix. Synaptopodin, a specific actin binding protein of the spine apparatus, is essential for its formation, but the underlying mechanisms remain unknown. We show that synaptopodin, when expressed in fibroblasts, forms actin-rich structures with connections to the ER, and that an ER-tethered synaptopodin assembles into liquid condensates. We also identified protein neighbors of synaptopodin in spines by in vivo proximity biotinylation. We validated a small subset of such proteins and showed that they co-assemble with synaptopodin in living cells. One of them is Pdlim7, an actin binding protein not previously identified in spines, and we show its precise colocalization with synaptopodin. We suggest that the matrix of the spine apparatus has the property of a liquid protein condensate generated by a multiplicity of low affinity interactions. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=131 SRC="FIGDIR/small/474557v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@197d23borg.highwire.dtl.DTLVardef@6b60eeorg.highwire.dtl.DTLVardef@1a8ce67org.highwire.dtl.DTLVardef@186cd0a_HPS_FORMAT_FIGEXP M_FIG C_FIG