Renaming the ‘OS-D/CSP’ Family (Part 1): ‘4-Cysteine Soluble Proteins’ (4CSPs)—Molecular Nomenclature, Structure, Expression, Evolution, Tissue-Distribution, and Pleiotropy

Chemosensory proteins (CSPs) are found in the olfactory sensory organs (antennae and maxillary palps) and/or gustatory sensory organs (labellum and legs) and have long been accepted to function through the binding of odorants. However, the same CSPs are also expressed in many tissues other than olfactory and gustatory organs, such as the gut, brain, fat body, wing, epidermis, <i>Corpora allata</i>, salivary gland, pheromone gland, prothoracic gland, etc. In this report, we suggest renaming the "chemosensory protein (CSP)" the "4-Cysteine Soluble Protein (4CSP)". This paradigm and nomenclature shift is based on molecular characteristics, genomic mining, tissue distribution, and functional roles beyond those related to olfaction. We examined prior studies on this protein gene family to bolster the renaming, highlighting the most recent findings that we ascribe to "pleiotropic properties" and evolutionary relevance rather than smell. The scope of the report, per se, is broad, and this is especially true given the volume of data that has been gathered on 4CSP expressed in ways that are not consistent with the olfactory paradigm. Statements outlining the many chemosensory properties of 4CSPs, particularly how they activate olfactory receptor neurons (ORNs), are currently scarce, if they exist at all. Many debates currently focus on 4CSPs' non-chemosensory functions, which are backed by a multitude of evidence, from gene evolution to tissue distribution. Therefore, strong arguments in favor of renaming chemosensory proteins are becoming evident here, outweighing the drawbacks.