Functional diversification and evolution of antifreeze proteins in the antarctic fish Lycodichthys dearborni.
Antifreeze proteins (AFPs) have independently evolved in many organisms. AFPs act by binding to ice crystals, effectively lowering the freezing point. AFPs are often at high copy number in a genome and diversity exists between copies. Type III antifreeze proteins are found in Arctic and Antarctic eel pouts, and have previously been shown to evolve under positive selection. Here we combine molecular and proteomic techniques to understand the molecular evolution and diversity of Type III antifreeze proteins in a single individual Antarctic fish Lycodichthys dearborni. Our expressed sequence tag (EST) screen reveals that at least seven different AFP variants are transcribed, which are ultimately translated into five different protein isoforms. The isoforms have identical 66 base pair signal sequences and different numbers of subsequent ice-binding domains followed by a stop codon. Isoforms with one ice-binding unit (monomer), two units (dimer), and multiple units (multimer) were present in the EST library. We identify a previously uncharacterized protein dimer, providing further evidence that there is diversity between Type III AFP isoforms, perhaps driven by positive selection for greater thermal hysteresis. Proteomic analysis confirms that several of these isoforms are translated and present in the liver. Our molecular evolution study shows that paralogs have diverged under positive selection. We hypothesize that antifreeze protein diversity is an important contributor to depressing the serum freezing point.
Kelley
Joanna L
JL
Department of Genome Sciences, University of Washington, Seattle, WA, USA. jlkelley@uchicago.edu
Aagaard
Jan E
JE
MacCoss
Michael J
MJ
Swanson
Willie J
WJ
D000595:Amino Acid Sequence; D000818:Animals; D000864:Antarctic Regions; D021301:Antifreeze Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D019143:Evolution, Molecular; D020224:Expressed Sequence Tags; D014644:Genetic Variation / Q000502:physiology; D016678:Genome; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010473:Perciformes / Q000235:genetics* / Q000378:metabolism; D017385:Sequence Homology
D021301:Antifreeze Proteins