Gene expression profiles of FABP genes in protochordates, Ciona intestinalis and Branchiostoma belcheri

Fatty-acid-binding proteins (FABPs) are small intracellular proteins associated with the transportation of fatty acids. Members of the FABPs share similar amino acid sequences and tertiary structures and form, together with a member of the cellular retinol-binding proteins (CRBPs), the intracellular-lipid-binding protein (iLBP) family. In vertebrates, several types of FABP have been isolated and classified into three subfamilies: 2-4. In invertebrates, several FABP-related proteins have been reported in protostomes and amphioxus; however, little is known about the relationship between their phylogenetic positions and expression patterns. We have performed a genome-wide survey of FABP-related genes in protochordates: amphioxus Branchiostoma belcheri and the ascidian Ciona intestinalis. Comprehensive BLAST searches in NCBI and the Ciona Ghost Database by using amino acid sequences of all FABPs have revealed that the ascidian C. intestinalis and amphioxus B. belcheri contain six and seven FABP-related genes in their haploid genomes, respectively. Expression pattern analyses by whole-mount in situ hybridization in Ciona transparent juveniles and serial-section in situ hybridizations in adult amphioxus have revealed that all genes are mainly expressed in the postpharyngeal digestive tract. In particular, the expression of FABP-related genes of subfamily-2 (liver/ileum type) and subfamily-3 (intestinal type) in the ascidian pyloric gland and amphioxus hepatic cecum provides insight into the evolution of hepatic-related structures of chordates and FABP-related genes.