mpp5am520 mutants partially lacks the N-terminal GUK domain of the Mpp5a protein. Mpp5a is a membrane-associated guanylate kinase protein which contains an evolutionary conserved region 1 (ECR1), a bipartite L27 domain (L27N, L27C), PDZ, SH3, 4.1, and guanylate kinase (GUK) domains. The mpp5am520 allele carries single base pair substitution that introduces stop codons within the GUK domain, causing a C-terminal truncation of the GUK domain.