A Mval PCR-RFLP detecting a silent allele at the goat alpha-lactalbumin locus

Alpha-lactalbumin (α-la), a calcium metalloprotein, is one of the major serum-proteins in ruminant milk (Jenness, 1982) and induces lactose synthesis in the mammary gland by interacting with the enzyme UDP-galactosyltransferase, giving rise to the heterodimer enzyme lactose synthase (Ebner & Brodbeck, 1968; Kuhn, 1983). The goat α-la transcription unit (LALBA), located on chromosome 5 (Hayes et al. 1993), is organized in 4 exons varying in length from 75 nucleotides (3rd exon) to 329 nucleotides (4th exon) coding for a 123-amino acid polypeptide chain (Vilotte et al. 1991). According to the strong similarity between bovine α-la (Vilotte et al. 1987) and human lysozyme (similar molecular weight, the same number of S-S bonds, identical N and C terminal residues; Peters et al. 1989), it has been proposed that both genes arose from a common ancestor (Vilotte et al. 1991).

A label selection procedure for determining the location of protein-protein interaction sites by cross-linking with bisimidoesters. Application to lactose synthase

A procedure is described that is designed to identify the primary site of cross-linking by bisimidoesters of a component of an interacting protein system. It is based on the mutually exclusive nature of acetylation and amidination. The procedure has been applied to the regulatory protein of lactose synthase, alpha-lactalbumin. A sample of bovine alpha-lactalbumin was acetylated with a trace amount of high specific activity [3H]acetic anhydride to produce a population of protein molecules essentially all of which contain 0 to 1 acetyl group; partial labeling of all 13 amino groups was obtained. This material was mixed with bovine colostrum galactosyltransferase in the presence of Mn2+, UDP-glucose, and N-acetylglucosamine, at pH 8.0, to promote complex formation and was cross-linked with dimethyl 3,3'-dithiobispropionimidate. Covalently cross-linked alpha-lactalbumin-galactosyltransferase (1:1) complex with characteristic enzymic and other properties was purified from the reaction mixture, and the distribution of [3H]acetyl label on each amino group of the alpha-lactalbumin component was determined, using procedures similar to those described in previous differential labeling studies (Richardson, R., and Brew, K. (1980) J. Biol. Chem. 255, 3377-3385). In comparison with the original labeled sample used for cross-linking, the specific activity of tritium label in 10 amino groups showed little change, whereas the labeling of three groups was changed markedly. The acetyl moiety on the epsilon-amino groups of lysines 5 and 108 showed major decreases in specific activity while that of lysine 114 was greatly increased. Similar results were obtained when the cross-linking was performed under different conditions of temperature, and cross-linker concentration. As the changes in lysines 5 and 114 are similar to those observed in differential labeling, they are attributed to alterations in the affinity for galactosyltransferase resulting from acetylation of these groups. In contrast, lysine 108, which is not sufficiently close to the interaction site to be perturbed in differential labeling studies but is greatly decreased in tritium content in the cross-linked complex, appears to represent the major site through which alpha-lactalbumin is cross-linked to galactosyltransferase as a result of the exclusion of protein molecules acetylated in this position from covalent cross-linking. Studies with a homologous series of bisimidoesters indicate that lysine 108 is situated 6.1 to 7.3 A degrees from an amino group on galactosyltransferase in the cross-linked complex. The general utility of the procedure and the nature of the interaction site in lactose synthase are discussed.

Lipoprotein particles from the Golgi apparatus of guinea-pig liver

1. A cell fraction has been isolated from guinea-pig liver and shown to be rich in Golgi apparatus by electron microscopy. The activity of UDP-d-galactose-N-acetylglucosamine galactosyltransferase was over 100-fold greater in this cell fraction than in the liver homogenate. These data support the conclusion that the fraction was enriched in Golgi apparatus. 2. The Golgi cisternae and secretory vesicles contained electron-dense particles of 10-80nm diameter. Disruption of the Golgi apparatus cell fraction released these particles, which were separated into VLD (very-low-density) and LD (low-density) species on the basis of their density. 3. The Golgi VLD particles possessed morphological, flotational, chemical and immunochemical properties which closely resembled those of the serum VLD lipoproteins from the same animals. 4. The Golgi LD particles were rich in phospholipid, containing 48.1% by weight. The chemical composition of these particles was quite distinct from that of the serum LD lipoproteins, but did, however, show some similarity to that of the serum VLD lipoproteins. A marked resemblance was noted in the chemical characteristics of the Golgi LD and VLD particles (with the exception of triglyceride content). In addition, these two species of Golgi particles possessed the same antigenic determinant. 5. The results suggest that the Golgi VLD particles are the precursors of the serum VLD lipoproteins. On the basis of similarities in gross chemical composition and in the antigenic determinant of the Golgi LD and VLD particles, we conclude that the LD particles are probably the precursors of the VLD particles. In view of the marked differences in gross chemical composition of the Golgi LD particles and serum LD lipoproteins, it appears unlikely that the LD particles are directly secreted into the serum pool.

Studies on the particulate lactose synthetase of mouse mammary gland and the role of -lactalbumin in the initiation of lactose synthesis

1. Some of the kinetic properties of the particulate lactose synthetase of mouse mammary gland were investigated and shown to resemble those that have been reported for the soluble enzyme. Typical values for intact preparations were 2.3mum for the apparent K(m) for alpha-lactalbumin at 40mm-glucose and 1.7mm for the apparent K(m) for glucose at the endogenous concentration of alpha-lactalbumin. 2. Digitonin treatment increased total assayable activity approximately twofold but almost eliminated the endogenous activity found in the absence of alpha-lactalbumin, these findings being consistent with the location of endogenous activity within Golgi vesicles. 3. From the properties of the particulate fraction from lactating mice it was deduced that the effective endogenous alpha-lactalbumin concentrations were in the range 1-10mum. 4. The concentration of alpha-lactalbumin was not significantly different in particles isolated at various stages of pregnancy and early lactation. 5. The implications of these results for the control of lactose synthetase in vivo are discussed.