Joseph C. Murphy

The vitamin content of oysters.

A twofold interest is connected with a study of the vitamin content of oysters. They constitute an important and an extensively used item of food. Furthermore, the material upon which they feed consists largely of diatoms and minute organisms, marine forms of life to which have been traced the origin of the fat-soluble vitamins found so abundantly in certain fish liver oils, such as that of the cod. 1 So far as we are aware, no work has been hitherto reported on the vitamin content of oysters with the exception of that published by Randoin 2 on the antiscorbutic factor. They found that oysters contain this vitamin in abundance. By means of feeding tests with rats, we have found that oysters are rich also in vitamins A and B In order to obtain uniform samples, fresh oysters were ground in a frozen condition. Quantities of the frozen product equivalent to 0.5 gm., calculated on a dry basis, caused prompt resumption of growth when fed daily to rats that had declined in weight as a result of the lack of vitamin B in their basal ration. Experiments in progress indicate that smaller quantities are sufficient to meet the requirements of rats for this vitamin. As little as 0.25 gm. of a product obtained by dehydrating fresh oysters at a temperature not exceeding 40° under reduced pressure, enabled rats to make a fair recovery from the results of vitamin A deficiency. It mas found that during the process of dehydrating the oysters a change took place which caused a partial destruction of vitamin B. Whether this process also impaired the vitamin A value of the oysters is being investigated. Work is also in progress to estimate the vitamin content of clams, shrimp and other articles of sea food.

Palmitoylation of Progressive Rod-Cone Degeneration (PRCD) Regulates Protein Stability and Localization.

Progressive rod-cone degeneration (PRCD) is a photoreceptor outer segment (OS) disc-specific protein with unknown function that is associated with retinitis pigmentosa (RP). The most common mutation in PRCD linked with severe RP phenotype is substitution of the only cysteine to tyrosine (C2Y). In this study, we find that PRCD is post-translationally modified by a palmitoyl lipid group at the cysteine residue linked with RP. Disrupting PRCD palmitoylation either chemically or by genetically eliminating the modified cysteine dramatically affects the stability of PRCD. Furthermore, in vivo electroporation of PRCD C2Y mutant in the mouse retina demonstrates that the palmitoylation of PRCD is important for its proper localization in the photoreceptor OS. Mutant PRCD C2Y was found in the inner segment in contrast to normal localization of WT PRCD in the OS. Our results also suggest that zDHHC3, a palmitoyl acyltransferase (PAT), catalyzes the palmitoylation of PRCD in the Golgi compartment. In conclusion, we find that the palmitoylation of PRCD is crucial for its trafficking to the photoreceptor OS and mislocalization of this protein likely leads to RP-related phenotypes.

Farnesylation of the Transducin G Protein Gamma Subunit Is a Prerequisite for Its Ciliary Targeting in Rod Photoreceptors

Primary cilia are microtubule-based organelles, which protrude from the plasma membrane and receive a wide range of extracellular signals. Various cilia use G protein-coupled receptors (GPCRs) for the detection of these signals. For instance, vertebrate rod photoreceptors use their cilia (also called outer segments) as antennae detecting photons by GPCR rhodopsin. Rhodopsin recognizes incoming light and activates its G protein, transducin, which is composed of three subunits α, β, and γ. Similar to all G protein γ subunits, the transducin Gγ1 subunit undergoes C-terminal prenylation resulting in the addition of an isoprenoid farnesyl; however, the significance of this posttranslational modification is unclear. To study the role of the farnesyl group, we genetically introduced a mutant Gγ1 that lacked the prenylation site into the retinal photoreceptors of mice. The biochemical and physiological analyses of these mice revealed that mutant Gγ1 dimerizes with the endogenous transducin Gβ1 subunit and that the resulting Gβγ dimers display reduced hydrophobicity. Although mutant Gβγ dimers could form a heterotrimeric G protein, they could not mediate phototransduction. This deficiency was due to a strong exclusion of non-farnesylated Gβγ complexes from the cilia (rod outer segments). Our results provide the first evidence that farnesylation is required for trafficking of G-protein βγ subunits to the cilium of rod photoreceptors.