Nanoscale distribution of proteins and their relative positioning within a defined subcellular region are key to their physiological functions. Thanks to the super-resolution imaging methods, especially single-molecule localization microscopy (SMLM), mapping the three-dimensional distribution of multiple proteins has been easier and more efficient than ever. Nevertheless, in spite of the many tools available for efficient localization detection and image rendering, it has been a challenge to quantitatively analyze the 3D distribution and relative positioning of proteins in these SMLM data. Here, using heterogeneously distributed synaptic proteins as examples, we describe in detail a series of analytical methods including detection of nanoscale density clusters, quantification of the trans-synaptic alignment between these protein densities, and automatic en face projection and averaging. These analyses were performed within customized Matlab routines and we make the full scripts available. The concepts behind these analytical methods and the scripts can be adapted for quantitative analysis of spatial organization of other macromolecular complexes.

Proteins display the capacity for adaptation to new functions, a property critical for evolvability. But what structural principles underlie the capacity for adaptation? Here, we show that adaptation to a physiologically distinct class of ligand specificity in a PSD95, DLG1, ZO-1 (PDZ) domain preferentially occurs through class-bridging intermediate mutations located distant from the ligand-binding site. These mutations provide a functional link between ligand classes and demonstrate the principle of \"conditional neutrality\" in mediating evolutionary adaptation. Structures show that class-bridging mutations work allosterically to open up conformational plasticity at the active site, permitting novel functions while retaining existing function. More generally, the class-bridging phenotype arises from mutations in an evolutionarily conserved network of coevolving amino acids in the PDZ family (the sector) that connects the active site to distant surface sites. These findings introduce the concept that allostery in proteins could have its origins not in protein function but in the capacity to adapt.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by high heritability. Recently, autism, the most profound form of ASD, has been increasingly attributed to synaptic abnormalities. Postsynaptic density 95 (PSD95), encoding PSD protein-95, was found essential for synaptic formation, maturation and plasticity at a PSD of excitatory synapse. It is possibly a crucial candidate gene for the pathogenesis of ASD. To identify the relationship between the rs13331 of PSD95 gene and ASD, we performed a case-control study in 212 patients and 636 controls in a Chinese population by using a polymerase chain reaction-restriction fragment length polymerase (PCR-RFLP) assay. The results showed that in genetic analysis of the heterozygous model, an association between the T allele of the rs13331 and ASD was found in the dominant model (OR=1.709, 95% CI 1.227-2.382, P=0.002) and the additive model (OR=1.409, 95% CI=1.104-1.800, P=0.006). Our data indicate that the genetic mutation C>T at the rs13331 in the PSD95 gene is strikingly associated with an increased risk of ASD.