The Muller F element (4.2 Mb, ~80 protein-coding genes) is an unusual autosome of Drosophila melanogaster; it is mostly heterochromatic with a low recombination rate. To investigate how these properties impact the evolution of repeats and genes, we manually improved the sequence and annotated the genes on the D. erecta, D. mojavensis, and D. grimshawi F elements and euchromatic domains from the Muller D element. We find that F elements have greater transposon density (25–50%) than euchromatic reference regions (3–11%). Among the F elements, D. grimshawi has the lowest transposon density (particularly DINE-1: 2% vs. 11–27%). F element genes have larger coding spans, more coding exons, larger introns, and lower codon bias. Comparison of the Effective Number of Codons with the Codon Adaptation Index shows that, in contrast to the other species, codon bias in D. grimshawi F element genes can be attributed primarily to selection instead of mutational biases, suggesting that density and types of transposons affect the degree of local heterochromatin formation. F element genes have lower estimated DNA melting temperatures than D element genes, potentially facilitating transcription through heterochromatin. Most F element genes (~90%) have remained on that element, but the F element has smaller syntenic blocks than genome averages (3.4–3.6 vs. 8.4–8.8 genes per block), indicating greater rates of inversion despite lower rates of recombination. Overall, the F element has maintained characteristics that are distinct from other autosomes in the Drosophila lineage, illuminating the constraints imposed by a heterochromatic milieu.

Contactin-associated protein-like 2 (CNTNAP2) encodes for CASPR2, a multidomain single transmembrane protein belonging to the neurexin superfamily that has been implicated in a broad range of human phenotypes including autism and language impairment. Using a combination of biophysical techniques, including small angle x-ray scattering, single particle electron microscopy, analytical ultracentrifugation, and biolayer interferometry, we present novel structural and functional data that relate the architecture of the extracellular domain of CASPR2 to a previously unknown ligand, Contactin1 (CNTN1). Structurally, CASPR2 is highly glycosylated and has an overall compact architecture. Functionally, we show that CASPR2 associates with micromolar affinity with CNTN1 but, under the same conditions, it does not interact with any of the other members of the contactin family. Moreover, by using dissociated hippocampal neurons we show that microbeads loaded with CASPR2, but not with a deletion mutant, co-localize with transfected CNTN1, suggesting that CNTN1 is an endogenous ligand for CASPR2. These data provide novel insights into the structure and function of CASPR2, suggesting a complex role of CASPR2 in the nervous system. Contactin-associated protein-like 2 (CASPR2)6 is a neuronal cell adhesion molecule known in rodents to be necessary for the clustering of the Kv1 potassium channels at juxtaparanodes (1). In myelinated nerves, CASPR2 is confined to the juxtaparanodal region of the axon where it appears to associate with the immunoglobulin domains of TAG-1 (transient axonal glycoprotein-1) to form a scaffold, which clusters the potassium channels Kv1.1 and Kv1.2 (2-4).CASPR2 is predicted to be a type I transmembrane protein of 1331 amino acids with the extracellular domain followed by a single transmembrane domain and a short (48 residues) intracellular domain that terminates with a class II PDZ recognition motif. Computational predictions suggest that CASPR2 has 12 putative N-linked glycosylation sites and 36 Cys residues likely making 18 disulfide bonds, forming 8 independently folded domains: four laminin, neurexin, sex hormone-binding globulin domains (LNS), two epidermal growth factor (EGF) domains, one discoidin domain, and one fibrinogen-like domain (Fig. 1A). CASPR2 shares an overall domain organization with ␣-neurexin-1 despite a relatively low amino acid identity (ϳ23% identity, ϳ39% similarity). However, distinctive features such as a discoidin domain in place of the first LNS domain and a fibrinogen-like domain in place of the 4th LNS domain suggest a different overall structural architecture. No information about the three-dimensional structure of CASPR2, other than that inferred from sequence homology, is currently available. Functionally, only TAG-1 (contactin 2 or CNTN2) has been thus far identified as the extracellular ligand for CASPR2 (2-4).Individuals in a cohort of Amish children, homozygous for a frameshift mutation (single-base G deletion at nucleotide 3709 in exon 22) involving the CNTNAP2 gene, ...

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that affects brain development, social and communication skills. Although the causes of the disease are mostly unknown rare variations of the CANTNAP2, the gene that codes for contactin‐associated protein‐like 2 (CASPR2), have been implicated in ASD. We compared the cellular localization and glycosylation processing of wild type CASPR2 to twelve CASPR2 point mutations found in autistic individuals. In contrast to the wild‐type protein that localizes to the cell surface, mutants showed altered cellular disposition with endoplasmic reticulum retention and activation of one signaling pathway of the unfolded protein response. As CASPR2 is associated with a set of neuropsychiatric diseases, it is of interest to study the interaction of CASPR2 with proteins such as the transient axonal glycoprotein‐1 (TAG1). We are currently studying the interaction between different CASPR2 mutants and TAG1 by various biochemical techniques. Preliminary results show that we can measure the interaction between wild‐type TAG1 and wild type CASPR2, but not with one CASPR2 mutants. This data suggest that the interaction is disrupted by the mutation but also that another protein could be mediating this interaction. These studies will help understand better the biology of CASPR2 and its specific role in the development of autism and other neurodevelopmental disorders.