Disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase elucidated by phosphorylation with protein kinase A in vitro and in tissue culture

Processing of human intestinal prolactase to an intermediate form by furin or by a furin-like proprotein convertase

Human lactase-phlorizin hydrolase (human-LPH) is synthesized as a large precursor (prepro-LPH), then cleaved to a pro-LPH of 220 kDa which is further cut to a "mature-like LPH" of a size close to that of mature LPH, i.e. about 150 kDa (in the processing of rabbit pro-LPH the intermediate has a mass of approximately 180 kDa). By coexpression of human prepro-LPH with furin in COS-7 cells we show that furin generates a mature-like LPH. Radioactive amino acid sequence analysis reveals that furin recognizes the motif R-T-P-R832, a protein convertase consensus, to generate a NH2 terminus located 36 amino acids upstream of the NH2 terminal found in vivo at Ala869. This intermediate is ultimately cleaved to the mature LPH form by other proteases including the pancreatic ones. These data demonstrate that human pro-LPH, like the rabbit enzyme, is processed to the mature enzyme by furin or furin-like enzymes through at least an intermediate form that has, however, an apparent mass close to that of the mature enzyme.

Identification of functional domains of rat intestinal phospholipase B/lipase. Its cDNA cloning, expression, and tissue distribution

A cDNA encoding a rat intestinal Ca(2+)-independent phospholipase B/lipase (PLB/LIP) was cloned from an ileac mucosa cDNA library using a probe amplified by polymerase chain reaction based on the purified enzyme's sequence. PLB/LIP consists of an NH2-terminal signal peptide, four tandem repeats of about 350 amino acids each, and a hydrophobic domain near the COOH terminus. The enzyme purified previously was found to be derived from the second repeat part. To examine the function of each domain, the full-length PLB/LIP, individual repeats, and a protein lacking the COOH-terminal hydrophobic stretch were expressed in COS-7 cells. The results showed that the second repeat, but not the other repeats, had all the activities (phospholipase A2, lysophospholipase, and lipase) found in the purified natural and expressed full-length enzymes, suggesting repeat 2 is a catalytic domain. The full-length enzyme was mainly present in membrane fractions and efficiently solubilized by treatment with 1% Triton X-100, but not with phosphatidylinositol-specific phospholipase C. Deletion of the COOH-terminal hydrophobic stretch caused the secretion of > 90% of synthesized PLB/LIP into culture media. These results suggest the hydrophobic domain is not replaced by a glycosylphosphatidylinositol anchor but serves as a membrane anchor directly. A message of the full-length PLB/LIP was abundantly expressed in the ileum and also, in a smaller, but significant amount, in the esophagus and testis. Immunohistochemistry showed that PLB/LIP is localized in brush border membranes of the absorptive cells, Paneth cells, and acrosomes of spermatid, suggesting its roles related and unrelated to intestinal digestion.

Functional diversity and interactions between the repeat domains of rat intestinal lactase

Lactase-phlorizin hydrolase (LPH), a major digestive enzyme in the small intestine of newborns, is synthesized as a high-molecular-mass precursor comprising four tandemly repeated domains. Proteolytic cleavage of the precursor liberates the pro segment (LPHalpha) corresponding to domains I and II and devoid of known enzymic function. The mature enzyme (LPHbeta) comprises domains III and IV and is anchored in the brush border membrane via a C-terminal hydrophobic segment. To analyse the roles of the different domains of LPHalpha and LPHbeta, and the interactions between them, we have engineered a series of modified derivatives of the rat LPH precursor. These were expressed in cultured cells under the control of a cytomegalovirus promoter. The results show that recombinant LPHbeta harbouring both domains III and IV produces lactase activity. Neither domain III nor IV is alone sufficient to generate active enzyme, although the corresponding proteins are transport-competent. Tandem duplication of domains III or IV did not restore lactase activity, demonstrating the separate roles of both domains within LPHbeta. Further, the development of lactase activity did not require LPHalpha; however, LPHalpha potentiated the production of active LPHbeta but the individual LPHalpha subdomains I and II were unable to do so. Lactase activity and targeting required the C-terminal transmembrane anchor of LPH; this requirement was terminal transmembrane anchor or LPH; this requirement was not satisfied by the signal/anchor region of another digestive enzyme: sucrase-isomaltase. On the basis of this study we suggest that multiple levels of intramolecular interactions occur within the LPH precursor to produce the mature enzyme, and that the repeat domains of the precursor have distinct and specific functions in protein processing, substrate recognition and catalysis. We propose a functional model of LPHbeta in which substrate is channelled from an entry point located within domain II to the active site located in domain IV.

Identification of homologues of the mammalian intestinal lactase gene in non-mammals (birds and molluscs)

Mammalian intestinal lactase hydrolyses a variety of beta-glycosides and is processed from a precursor comprising four tandem domains exhibiting sequence similarity, suggestive of multiple duplication events in the evolutionary past. The aim of the present study was to investigate whether genes homologous to the lactase gene exist in animals other than mammals. A reverse transcriptase-PCR strategy using a degenerate mixture of oligonucleotides was developed to search for the presence of transcripts similar in sequence to the mammalian lactase mRNA in the digestive tracts of a bird (the chicken) and an invertebrate (the mussel). Partial cDNAs corresponding to the 3' end of intestinal mRNAs were identified in both animals. In chicken, two cDNAs were isolated, corresponding to 6.5 kb transcripts that used two distinct polyadenylation sites. In mussels, three cDNAs were obtained and classified into two categories. One class of cDNA hybridized to a major mRNA of 3.5 kb and to minor species of 4.5 kb and 6 kb. The second class of cDNA hybridized to a 13 kb transcript, which was approximately twice as large as the mammalian lactase mRNA. Peptide sequences predicted from the chicken and mussel cDNAs confirmed that the proteins are related to mammalian lactase. They also suggested that the chicken protein and one mussel protein are integral molecules anchored in the cell membrane by a C-terminal transmembrane anchor, like lactase. These data provide evidence that proteins phylogenetically related to the mammalian-specific lactase are widespread in the animal kingdom, and that these proteins are expressed in the intestinal tract.

Furin, PC1/3, and/or PC6A process rabbit, but not human, pro-lactase-phlorizin hydrolase to the 180-kDa intermediate

Small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a large precursor (prepro-LPH) of 1926 amino acids. In the endoplasmic reticulum, prepro-LPH is split by signal protease. The resulting pro-LPH is cut to mature LPH directly (human) or via a 180-kDa intermediate (rabbit), most likely in the trans-Golgi network or in a later compartment. Antibodies directed against different regions of rabbit pro-LPH locate the cleavage site resulting in the 180-kDa intermediate between amino acid residues 79 and 286. This stretch contains the two sequences -Arg-Cys-Tyr-Arg114 approximately -Arg-Ala-Ser-Arg191 approximately, which are potential cleavage sites for subtilisin-like proprotein convertases. These sites are not conserved in human pro-LPH. By coexpression in COS 7 cells of rabbit prepro-LPH and proprotein convertases (PC 1/3, PC2, PC6A, PC6B, furin), we show that furin, PC 1/3, and PC6A generate a processing intermediate that is immunologically indistinguishable from the one observed in vivo. Furin, PC 1/3, and PC6A are all expressed in the small intestine as shown by a polymerase chain reaction-based approach and, more specifically, in enterocytes, as shown by in situ hybridization. These results suggest that furin, PC 1/3, and/or PC6A are responsible for the in vivo processing of rabbit pro-LPH to the 180-kDa intermediate.

Disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase elucidated by phosphorylation with protein kinase A in vitro and in tissue culture

The intracellular disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase (LPH) is demonstrated, using a specific phosphorylation of Ser1916 by protein kinase A (PKA). This phosphorylation is shown to occur not only in vitro (with pure LPH and pure catalytic subunit of PKA), but also in an organ culture of the small intestine. Cholera toxin, which is known to act in vivo on the membranes of the small intestine, with severe clinical consequences, and to elevate the intracellular cyclic AMP of enterocytes, is shown to enhance significantly the phosphorylation of LPH in intact cells grown as an organ culture. These findings establish the cytosolic orientation of the carboxy-terminus tail of LPH in situ, and raise the possibility that the tail itself and its phosphorylation by PKA may have a physiological or physiopathological significance.