Mutations in the human cellular retinaldehyde binding protein (CRALBP) gene cause retinal pathology. To understand the molecular basis of impaired CRALBP function, we have characterized human recombinant CRALBP containing the disease causing mutations R233W or M225K. Protein structures were verified by amino acid analysis and mass spectrometry, retinoid binding properties were evaluated by UV-visible and fluorescence spectroscopy and substrate carrier functions were assayed for recombinant 11-cis-retinol dehydrogenase (rRDH5). The M225K mutant was less soluble than the R233W mutant and lacked retinoid binding capability and substrate carrier function. In contrast, the R233W mutant exhibited solubility comparable to wild type rCRALBP and bound stoichiometric amounts of 11-cis-and 9-cis-retinal with at least 2-fold higher affinity than wild type rCRALBP. Holo-R233W significantly decreased the apparent affinity of rRDH5 for 11-cis-retinoid relative to wild type rCRALBP. Analyses by heteronuclear single quantum correlation NMR demonstrated that the R233W protein exhibits a different conformation than wild type rCRALBP, including a different retinoid-binding pocket conformation. The R233W mutant also undergoes less extensive structural changes upon photoisomerization of bound ligand, suggesting a more constrained structure than that of the wild type protein. Overall, the results show that the M225K mutation abolishes and the R233W mutation tightens retinoid binding and both impair CRALBP function in the visual cycle as an 11-cisretinol acceptor and as a substrate carrier.Mutations in the human gene RLBP1 encoding the cellular retinaldehyde-binding protein (CRALBP) 1 cause retinal pathology and have been associated with autosomal recessive retinitis pigmentosa (1), Bothnia dystrophy (2, 3), retinitis punctata albescens (4), fundus albipunctatus (5), and Newfoundland rod-cone dystrophy (6). These diseased phenotypes are all characterized by photoreceptor degeneration and night blindness (delayed dark adaptation) but differ in age of onset, rate of progression, and severity. The molecular basis for the clinical differences in these related retinal dystrophies is not well understood and no effective therapies exist for the pathology resulting from impaired CRALBP function.CRALBP is an abundant, 36-kDa protein in the cytosol of the retinal pigment epithelium (RPE) and Mü ller cells of the retina where it carries endogenous 11-cis-retinol and 11-cis-retinal (7). The CRALBP ligand binding cavity is mapped in the accompanying report (8). In vivo studies (9) show that CRALBP serves as a major 11-cis-retinol acceptor in the isomerization step of the visual cycle (7, 10, 11), stimulating the enzymatic isomerization of all-trans-to 11-cis-retinol in the rod visual cycle. However, CRALBP appears to function within an RPE protein complex (12) and to serve multiple functions. In vitro, CRALBP facilitates the oxidation of 11-cis-retinol to 11-cisretinal by 11-cis-retinol dehydrogenase (12, 13), retards 11-cisretinol esterification...

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In this paper, we develop a novel response to counterfactual scepticism, the thesis that most ordinary counterfactual claims are false. In the process we aim to shed light on the relationship between debates in the philosophy of science and debates concerning the semantics and pragmatics of counterfactuals. We argue that science is concerned with many domains of inquiry, each with its own characteristic entities and regularities; moreover, statements of scientific law often include an implicit ceteris paribus clause that restricts the scope of the associated regularity to circumstances that are 'fitting' to the domain in question. This observation reveals a way of responding to scepticism while, at the same time, doing justice both to the role of counterfactuals in science and to the complexities inherent in ordinary counterfactual discourse and reasoning. Keywords Counterfactuals • Counterfactual scepticism • Ceteris paribus laws • Contextualism • HájekCounterfactual scepticism, the thesis that most counterfactuals are false, has received a fair amount of attention recently. The discussion of counterfactual scepticism and how one should react to it, either by resisting it somehow or by accepting it and living with the consequences, yields insights into the nature and utility of counterfactuals and counterfactual reasoning.

In this paper, we argue that a distinction ought to be drawn between two ways in which a given world might be logically impossible. First, a world w might be impossible because the laws that hold at w are different from those that hold at some other world (say the actual world). Second, a world w might be impossible because the laws of logic that hold in some world (say the actual world) are violated at w. We develop a novel way of modelling logical possibility that makes room for both kinds of logical impossibility. Doing so has interesting implications for the relationship between logical possibility and other kinds of possibility (for example, metaphysical possibility) and implications for the necessity or contingency of the laws of logic.

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