Activation of Jak tyrosine kinases through hematopoietic cytokine receptors occurs as a consequence of ligand-induced aggregation of receptor-associated Jaks and their subsequent autophosphorylation. Jak kinases consist of a C-terminal tyrosine kinase domain, a pseudokinase domain of unknown function, and Jak homology (JH) domains 3 to 7, implicated in receptor-Jak interaction. We analyzed the functional roles of the different protein domains in activation of Jak2. Deletion analysis of Jak2 showed that the pseudokinase domain but not JH domains 3 to 7 negatively regulated the catalytic activity of Jak2 as well as Jak2-mediated activation of Stat5. Phosphorylation of Stat5 by wild-type Jak2 was dependent on the SH2 domain of Stat5; however, this requirement was lost upon deletion of the pseudokinase domain of Jak2. Investigation of the mechanisms of the pseudokinase domain-mediated inhibition of Jak2 suggested that this regulation did not involve protein tyrosine phosphatases. Instead, analysis of interactions between the tyrosine kinase domain and Jak2 suggested that the pseudokinase domain interacted with the kinase domain. Furthermore, coexpression of the pseudokinase domain inhibited the activity of the single tyrosine kinase domain. Finally, deletion of the pseudokinase domain of Jak2 deregulated signal transduction through the gamma interferon receptor by significantly increasing ligand-independent activation of Stat transcription factors. These results indicate that the pseudokinase domain negatively regulates the activity of Jak2, probably through an interaction with the kinase domain, and this regulation is required to keep Jak2 inactive in the absence of ligand stimulation. Furthermore, the pseudokinase domain may have a role in regulation of Jak2-substrate interactions.

Collagens carry hydroxylysine residues that act as attachment sites for carbohydrate units and are important for the stability of crosslinks but have been regarded as nonessential for vertebrate survival. We generated mice with targeted inactivation of the gene for one of the three lysyl hydroxylase isoenzymes, LH3. The null embryos developed seemingly normally until embryonic day 8.5, but development was then retarded, with death around embryonic day 9.5. Electron microscopy (EM) revealed fragmentation of basement membranes (BMs), and immuno-EM detected type IV collagen within the dilated endoplasmic reticulum and in extracellular aggregates, but the typical BM staining was absent. Amorphous intracellular and extracellular particles were also seen by collagen IV immunofluorescence. SDS͞PAGE analysis demonstrated increased mobilities of the type IV collagen chains, consistent with the absence of hydroxylysine residues and carbohydrates linked to them. These results demonstrate that LH3 is indispensable for biosynthesis of type IV collagen and for BM stability during early development and that loss of LH3's functions leads to embryonic lethality. We propose that the premature aggregation of collagen IV is due to the absence of the hydroxylysine-linked carbohydrates, which thus play an essential role in its supramolecular assembly. L ysyl hydroxylase (LH, EC 1.14.11.4, Plod) catalyzes the hydroxylation of lysine residues in -X-Lys-Gly-triplets in collagens and other proteins with collagen-like sequences (1). The hydroxylysine residues formed have two important functions: (i) they serve as attachment sites for carbohydrates, either monosaccharide galactose or disaccharide glucosylgalactose, and (ii) they have an important role in the stabilization of intermolecular collagen crosslinks that provide tensile strength and mechanical stability for the collagen fibrils. The extent of lysine hydroxylation and hydroxylysine glycosylation varies between collagen types. Nearly 90% of all lysines are hydroxylated and hydroxylysines glycosylated in types IV and VI collagen, whereas Ͻ20% of the lysines are hydroxylated in type III collagen (2). The functions of the hydroxylysine-linked carbohydrates are poorly understood, but in the case of fibril-forming collagens, they are thought to regulate the formation and morphology of the fibrils (1, 3).Three human LH isoenzymes have been identified (1, 4-6). Mutations in the LH1 gene lead to the kyphoscoliotic type of Ehlers-Danlos syndrome, which is characterized by scoliosis, joint laxity, skin fragility, ocular manifestations, and severe muscle hypotonia (7-9). Mutations in the LH2 gene have recently been reported in two families with Bruck syndrome, which is characterized by fragile bones, joint contractures, scoliosis, and osteoporosis (10).No mutations have been characterized in the gene for the isoenzyme LH3, and its in vivo functions have not yet been elucidated. This protein differs from the other two lysyl hydroxylase isoenzymes in that it also possesses relatively low levels...

We have generated mice with targeted inactivation of the Plod1 gene for lysyl hydroxylase 1 (LH1). Its human mutations cause Ehlers-Danlos syndrome VIA (EDS VIA) characterized by muscular hypotonia, joint laxity, and kyphoscoliosis. The Plod1 ؊/؊ mice are flaccid and have gait abnormalities. About 15% of them died because of aortic rupture and smooth muscle cells in non-ruptured Plod1 ؊/؊ aortas showed degenerative changes. Collagen fibrils in the Plod1 ؊/؊ aorta and skin had an abnormal morphology. The LH activity level in the Plod1 ؊/؊ skin and aorta samples was 35-45% of that in the wild type. The hydroxylysine content was decreased in all the Plod1 ؊/؊ tissues, ranging from 22% of that in the wild type in the skin to 75 and 86% in the femur and lung. The hydroxylysylpyridinoline crosslinks likewise showed decreases in all the Plod1 ؊/؊ tissues, ranging from 28 and 33% of that in the wild type in the aorta and cornea to 47 and 59% in femur and tendon, while lysylpyridinolines were increased. The hydroxylysines found in the Plod1 ؊/؊ collagens and their cross-links were evidently synthesized by the other two LH isoenzymes. Few data are available on abnormalities in EDS VIA tissues other than the skin. Plod1 ؊/؊ mice offer an in vivo model for systematic analysis of the tissue-specific consequences of the lack of LH1 activity and may also provide a tool for analyzing the roles of connective tissue in muscle function and the complex interactions occurring in the proper assembly of the extracellular matrix.Lysyl hydroxylase (LH, 2 EC 1.14.11.4) catalyzes the hydroxylation of lysine residues mainly but not exclusively in -X-LysGly-triplets in collagens and proteins with collagen-like sequences (1). The enzyme resides within the endoplasmic reticulum and has three human and mouse isoenzymes, LHs 1-3 (1-7). The hydroxylysine residues formed have two important functions: they are essential for the stability of the intermolecular cross-links that provide the collagen fibrils with their tensile strength and mechanical stability, and they serve as attachment sites for carbohydrate units, either the monosaccharide galactose or the disaccharide glucosylgalactose (1). Collagen cross-link formation occurs in the extracellular matrix and is initiated by the conversion of specific lysine or hydroxylysine residues in the telopeptides, i.e. the short non-triple helical ends of collagen molecules, into the aldehydes allysine or hydroxyallysine, respectively, catalyzed by lysyl oxidase (8). The telopeptides are connected with the triple helical part of an adjacent molecule by difunctional immature cross-links in the characteristic staggered array of collagen molecules in a fibril. The three main fibril-forming collagens, types I, II, and III, have four cross-linking sites, one in each telopeptide and two in the triple helical region, close to its N-and C-terminal ends (9). If the residue in the telopeptide is hydroxyallysine, the difunctional cross-links can mature into trifunctional non-reducible cross-links comprising lysylpyri...

Lysyl hydroxylase (LH) catalyzes the formation of hydroxylysine in collagens; three human isoenzymes have been cloned so far. We report here on the purification of all three recombinant isoenzymes to homogeneity from the medium of cultured insect cells, and we demonstrate that they are all homodimers. Limited proteolysis experiments identified two main protease-sensitive regions in the monomers of about 80 -85 kDa, corresponding to three fragments A-C (from the N to C terminus), with molecular masses of about 30, 37, and 16 kDa, respectively. Fragment A was found to play no role in LH activity as a recombinant B-C polypeptide constituted a fully active hydroxylase with K m values for cosubstrates and the peptide substrate that were identical to those of the full-length enzyme. LH3, but not LH1 and LH2, has also been reported recently (Heikkinen, J., Risteli, M., Wang, C., Latvala, J., Rossi, M., Valtavaara, M., and Myllylä , R. (2000) J. Biol. Chem. 275, 36158 -36163) to possess collagen glucosyltransferase activity. We confirm this highly surprising finding here and extend it by demonstrating that LH3 may also possess trace amounts of collagen galactosyltransferase activity. All the glucosyltransferase and galactosyltransferase activity of LH3 was found to reside in fragment A, which played no role in the hydroxylase activity of the polypeptide. This fragment is about 55% identical and 80% similar to the corresponding fragments of LH1 and LH2. However, the levels of the glycosyltransferase activities are so low that they may be of little biological significance. It is thus evident that human tissues must have additional glycosyltransferases that are responsible for most of the collagen glycosylation in vivo.