OBP

Chemical communication is mediated by sex-biased signals abundantly present in the urine, saliva and tears. Because most studies concentrated on the urinary signals, we aimed to determine the saliva proteome in wild Mus musculus musculus, to extend the knowledge on potential roles of saliva in chemical communication. We performed the gel-free quantitative LC-MS/MS analyses of saliva and identified 633 proteins with 134 (21%) of them being sexually dimorphic. They include proteins that protect and transport volatile organic compounds in their beta barrel including LCN lipocalins, major urinary proteins (MUPs), and odorant binding proteins (OBPs). To our surprise, the saliva proteome contains one MUP that is female biased (MUP8) and the two protein pheromones MUP20 (or 'Darcin') and ESP1 in individuals of both sex. Thus, contrary to previous assumptions, our findings reveal that these proteins cannot function as male-unique signals. Our study also demonstrates that many olfactory proteins (e.g. LCNs, and OBPs) are not expressed by submandibular glands but are produced elsewhere–in nasal and lacrimal tissues, and potentially also in other oro-facial glands. We have also detected abundant proteins that are involved in wound healing, immune and non-immune responses to pathogens, thus corroborating that saliva has important protective roles. The sequence of the mouse genome provided a tool to study blueprints for all RNAs and proteins in mice 1. Progenitors of modern laboratory mice were hybrids among Mus musculus domesticus, Mus musculus musculus and other subspecies. Though, laboratory mice have been widely and successfully used as experimental organisms in studies of biomarkers of physiological states 2 and of human pathological conditions, they may be less suitable to study chemical communication, a process which is driven by sexual selection. This is due to the differential contribution of blocks of genes from the two house-mouse subspecies M. m. domesticus and M. m. musculus to current laboratory strains 1 that may mask typical intra-and inter-specific differences. One of our aims, therefore, was to define the saliva proteome in M. m. musculus to provide the array of proteins and their quantity characteristic for saliva in a wild living mouse species. This study also represents a baseline for future comparative studies focusing on chemical communication and immunity. In mice, the most published studies in chemical communication focused on the major urinary proteins (MUPs) 3–7 , which are expressed by the liver and transport volatile organic compounds (VOCs) in their beta barrel structure to the urine 6–11. VOCs are slowly released from different urinary MUPs, and have been proposed to function in a variety of social signals, including identity, territorial marking, mate choice etc. 3,12–14. Thus, lipoca-lins and their specific ligands together form a signal 15. Differential ligand binding may have a potential influence on sub-species recognition between M. m. musculus and M. m. domesticus 16–18. These two sub-species have been previously shown to vary in the abundance of male VOCs 19 and in MUP expression between the two subspecies and individuals of the opposite sex 9. Moreover, scent signals have been shown to be an integral part of subspecies recognition and could play important roles in preventing interspecific mating between the two house mouse subspecies 18. Increasing number of papers, however, show that MUP expression is linked to reproduction and sociality, and not just to competitive ability 3,4,20–23. The expression of urinary MUPs is socially regulated in that males excrete