Structural and biophysical characterization of protocadherin extracellular regions

Neural circuit assembly requires that the axons and dendrites of the same neuron do not overlap each other while interacting freely with those from different neurons. This requires that each neuron have a unique cell surface identity to that of its neighbors and that neural self-recognition leads to repulsion, a process known as self-avoidance. Self-avoidance is perhaps best understood in Drosophilia, where homophilic recognition between individual Dscam1 isoforms on the cell surface of neurons leads to repulsion between sister dendrites and axons. However, in contrast to Drosophila, where alternative splicing of the Dscam1 gene can generate thousands of isoforms, vertebrate Dscam genes do not generate significant diversity. The most promising candidate to fill this role in vertebrates is the clustered protocadherins (Pcdhs). Despite this hypothesis, little is known about clustered Pcdh proteins and how they interact. The clustered Pcdh genes are encoded in three contiguous gene loci, Pcdha, Pcdhb, and Pcdhg, which encode three related families of proteins, Pcdhα, -β, and

Single-Cell Identity Generated by Combinatorial Homophilic Interactions between α, β, and γ Protocadherins

Individual mammalian neurons express distinct repertoires of protocadherin (Pcdh) -α, -β and -γ proteins that function in neural circuit assembly. Here we show that all three types of Pcdhs can engage in specific homophilic interactions, that cell surface delivery of alternate Pcdhα isoforms requires cis interactions with other Pcdh isoforms, and that the extracellular cadherin domain EC6 plays a critical role in this process. Analysis of specific combinations of up to five Pcdh isoforms showed that Pcdh homophilic recognition specificities strictly depend on the identity of all of the expressed isoforms, such that mismatched isoforms interfere with cell-cell interactions. We present a theoretical analysis showing that the assembly of Pcdh-α, −β and −γ isoforms into multimeric recognition units, and the observed tolerance for mismatched isoforms can generate the cell surface diversity necessary for single-cell identity. However, competing demands of non-self discrimination and self-recognition place limitations on the mechanisms by which recognition units can function

Molecular Logic of Neuronal Self-Recognition through Protocadherin Domain Interactions

SummarySelf-avoidance, a process preventing interactions of axons and dendrites from the same neuron during development, is mediated in vertebrates through the stochastic single-neuron expression of clustered protocadherin protein isoforms. Extracellular cadherin (EC) domains mediate isoform-specific homophilic binding between cells, conferring cell recognition through a poorly understood mechanism. Here, we report crystal structures for the EC1–EC3 domain regions from four protocadherin isoforms representing the α, β, and γ subfamilies. All are rod shaped and monomeric in solution. Biophysical measurements, cell aggregation assays, and computational docking reveal that trans binding between cells depends on the EC1–EC4 domains, which interact in an antiparallel orientation. We also show that the EC6 domains are required for the formation of cis-dimers. Overall, our results are consistent with a model in which protocadherin cis-dimers engage in a head-to-tail interaction between EC1–EC4 domains from apposed cell surfaces, possibly forming a zipper-like protein assembly, and thus providing a size-dependent self-recognition mechanism

Yeast Rap1 contributes to genomic integrity by activating DNA damage repair genes

Rap1 (repressor-activator protein 1) is a multifunctional protein that controls telomere function, silencing and the activation of glycolytic and ribosomal protein genes. We have identified a novel function for Rap1, regulating the ribonucleotide reductase (RNR) genes that are required for DNA repair and telomere expansion. Both the C terminus and DNA-binding domain of Rap1 are required for the activation of the RNR genes, and the phenotypes of different Rap1 mutants suggest that it utilizes both regions to carry out distinct steps in the activation process. Recruitment of Rap1 to the RNR3 gene is dependent on activation of the DNA damage checkpoint and chromatin remodelling by SWI/SNF. The dependence on SWI/SNF for binding suggests that Rap1 acts after remodelling to prevent the repositioning of nucleosomes back to the repressed state. Furthermore, the recruitment of Rap1 requires TAFIIs, suggesting a role for TFIID in stabilizing activator binding in vivo. We propose that Rap1 acts as a rheostat controlling nucleotide pools in response to shortened telomeres and DNA damage, providing a mechanism for fine-tuning the RNR genes during checkpoint activation