Circulatory lipid transport in animals is mediated to a substantial extent by members of the large lipid transfer (LLT) protein (LLTP) superfamily. These proteins, including apolipoprotein B (apoB), bind lipids and constitute the structural basis for the assembly of lipoproteins. The current analyses of sequence data indicate that LLTPs are unique to animals and that these lipid binding proteins evolved in the earliest multicellular animals. In addition, two novel LLTPs were recognized in insects. Structural and phylogenetic analyses reveal three major families of LLTPs: the apoB-like LLTPs, the vitellogenin-like LLTPs, and the microsomal triglyceride transfer protein (MTP)-like LLTPs, or MTPs. The latter are ubiquitous, whereas the two other families are distributed differentially between animal groups. Besides similarities, remarkable variations are also found among LLTPs in their major lipid-binding sites (i.e., the LLT module as well as the predicted clusters of amphipathic secondary structure): variations such as protein modification and number, size, or occurrence of the clusters. Strikingly, comparative research has also highlighted a multitude of functions for LLTPs in addition to circulatory lipid transport. The integration of LLTP structure, function, and evolution reveals multiple adaptations, which have come about in part upon neofunctionalization of duplicated genes. Moreover, the change, exchange, and expansion of functions illustrate the opportune application of lipid-binding proteins in nature.Accordingly, comparative research exposes the structural and functional adaptations in animal lipid carriers and brings up novel possibilities for the manipulation of lipid transport.

Prion protein (PrP) sequences are until now available for only six of the 18 orders of placental mammals. A broader comparison of mammalian prions might help to understand the enigmatic functional and pathogenic properties of this protein. We therefore determined PrP coding sequences in 26 mammalian species to include all placental orders and major subordinal groups. Glycosylation sites, cysteines forming a disulfide bridge, and a hydrophobic transmembrane region are perfectly conserved. Also, the sequences responsible for secondary structure elements, for N- and C-terminal processing of the precursor protein, and for attachment of the glycosyl-phosphatidylinositol membrane anchor are well conserved. The N-terminal region of PrP generally contains five or six repeats of the sequence P(Q/H)GGG(G/-)WGQ, but alleles with two, four, and seven repeats were observed in some species. This suggests, together with the pattern of amino acid replacements in these repeats, the regular occurrence of repeat expansion and contraction. Histidines implicated in copper ion binding and a proline involved in 4-hydroxylation are lacking in some species, which questions their importance for normal functioning of cellular PrP. The finding in certain species of two or seven repeats, and of amino acid substitutions that have been related to human prion diseases, challenges the relevance of such mutations for prion pathology. The gene tree deduced from the PrP sequences largely agrees with the species tree, indicating that no major deviations occurred in the evolution of the prion gene in different placental lineages. In one species, the anteater, a prion pseudogene was present in addition to the active gene.

Lipoproteins mediate most of the lipid transport in the circulation of animals. In mammals, a single protein component, the nonexchangeable apolipoprotein B (apoB), provides the structural basis for the biosynthesis of neutral fattransporting lipoproteins (1, 2). Interestingly, the major lipoprotein of insects, lipophorin, contains two structural apolipoproteins, because the insect apoB homolog (3, 4), the precursor apolipophorin-II/I (apoLp-II/I), is cleaved during lipoprotein biosynthesis by the fat body (5, 6).Cleavage of apoLp-II/I can be related to the activity of furin, a member of the proprotein convertase (PC) family of subtilisin-like serine endoproteases that is mainly active in the trans -Golgi network (7). The preferred consensus substrate sequence for furin, R-X-K/R-R, is present in all apoLp-II/I sequences characterized to date (8-11). In agreement with the activity of furin, Locusta migratoria apoLp-II/I appears to be cleaved immediately C terminal of its furin substrate sequence, RQKR 720 , as indicated by the N-terminal sequence of apoLp-I (10).The predicted furin cleavage site in each insect apoLp-II/I is located in the large lipid transfer (LLT) domain, which constitutes the N-terminal region of apoLp-II/I that has sequence homology to that of apoB, the microsomal triglyceride transfer protein (MTP) large subunit, and vitellogenin (3, 4). In apoB, this domain is essential for lipoprotein biosynthesis. The interaction between the LLT domain of apoB and that of the MTP large subunit enables the assembly of apoB-containing lipoproteins (1, 2). The homology between apoB and apoLp-II/I, as well as the presence of an MTP large subunit in insects (12), suggest that the LLT domain of apoLp-II/I enables lipoprotein Abbreviations: apoB, apolipoprotein B; apoLp, apolipophorin; decRVKRcmk, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone; HDLp, high density lipophorin; LDLp, low density lipophorin; LDLR, low density lipoprotein receptor; LLT, large lipid transfer; MTP, microsomal triglyceride transfer protein; PC, proprotein convertase.

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By alternative use of four RSL (reactive site loop) coding exon cassettes, the serpin (serine protease inhibitor) gene Spn4 from Drosophila melanogaster was proposed to enable the synthesis of multiple protease inhibitor isoforms, one of which has been shown to be a potent inhibitor of human furin. Here, we have investigated the inhibitory spectrum of all Spn4 RSL variants. The analyses indicate that the Spn4 gene encodes inhibitors that may inhibit serine proteases of the subtilase family (S8), the chymotrypsin family (S1), and the papain-like cysteine protease family (C1), most of them at high rates. Thus a cohort of different protease inhibitors is generated simply by grafting enzyme-adapted RSL sequences on to a single serpin scaffold, even though the target proteases contain different types and/or a varying order of catalytic residues and are descendents of different phylogenetic lineages. Since all of the Spn4 RSL isoforms are produced as intracellular residents and additionally as variants destined for export or associated with the secretory pathway, the Spn4 gene represents a versatile defence tool kit that may provide multiple antiproteolytic functions.