Human leukocyte antigen (HLA)-DM is an unconventional major histocompatibility complex (MHC) class II heterodimer that is important for B-cell-mediated antigen processing and presentation to MHC class II-restricted T cells. HLA-DM is encoded by two genes, DMA and DMB, which map to the MHC class II region, and shares some homology with MHC class I and class II proteins. Here we define the biochemical role of HLA-DM. Recombinant soluble HLA-DM heterodimers have been purified from culture supernatants of insect cell transformants. At pH 5.0, they induce the dissociation of a subset of peptides bound to HLA-DR, including a nested set of class-II-associated invariant chain peptides (CLIP). This process liberates HLA-DR and leads to the enhanced binding of exogenous peptides.

The proteolytic enzyme stromelysin-1 is a member of the family of matrix metalloproteinases and is believed to play a role in pathological conditions such as arthritis and tumor invasion. Stromelysin-1 is synthesized as a proenzyme that is activated by removal of an N-terminal prodomain. The active enzyme contains a catalytic domain and a C-terminal hemopexin domain believed to participate in macromolecular substrate recognition. We have determined the three-dimensional structures of both a C-truncated form of the proenzyme and an inhibited complex of the catalytic domain by X-ray diffraction analysis. The catalytic core is very similar in the two forms and is similar to the homologous domain in fibroblast and neutrophil collagenases, as well as to the stromelysin structure determined by NMR. The prodomain is a separate folding unit containing three a-helices and an extended peptide that lies in the active site of the enzyme. Surprisingly, the amino-to-carboxyl direction of this peptide chain is opposite to that adopted by the inhibitor and by previously reported inhibitors of collagenase. Comparison of the active site of stromelysin with that of thermolysin reveals that most of the residues proposed to play significant roles in the enzymatic mechanism of thermolysin have equivalents in stromelysin, but that three residues implicated in the catalytic mechanism of thermolysin are not represented in stromelysin.

Interleukin 1 (IL-1) is a lymphokine secreted by monocytes in response to a variety of inflammatory stimuli. IL-1fB the predominant form of IL-1 produced by human monocytes, is synthesized as an inactive precursor of 31 kDa and is cleaved at Asp"'6-Ala"17 to yield a 17.5-kDa extracellular form. The exact cellular site of cleavage and mechanism of secretion is at present unknown. We have prepared cell-free postnuclear extracts from freshly isolated human monocytes as well as THP. (1) provided the first substantive evidence that, in a mouse monocyte cell line, IL-1 was synthesized as a cell-associated precursor that could be chased into an extracellular 17-kDa form. Subsequently, reports emerged which suggested that a 31-kDa form of IL-1i3 was associated with human monocytes (2-5) and that this material was cleaved to release the mature form (2, 4, 5). These studies were corroborated by cDNA sequence data from a number of species which indicated that IL-1 mRNA encodes a larger protein than that identified as mature secreted IL-1 (6-10). As precursor IL-183 (pre-IL-1,j)is unable to bind to IL-1 receptors and is biologically inactive (11), some form of proteolytic processing is apparently required to release active IL-1p8. While the kinetics of IL-1 synthesis and secretion has been analyzed in some detail, little has been uncovered about the mechanism by which IL-1 is synthesized, processed, and secreted. Analysis ofthe predicted amino acid sequence from pre-IL-1,3 cDNA has not revealed the presence of a unique hydrophobic signal sequence domain, common to most secreted proteins (6)(7)(8)(9)(10)12). The N-terminal amino acid of mature monocyte IL-1p from humans has been sequenced by a number of investigators as Ala"17 (6, 13), suggesting that a cleavage site exists between Asp'6 and Ala"7. While the first 116 residues may be considered a signal sequence of sorts, it is not recognized as such by otherwise competent endoplasmic reticulum membranes (G.L., unpublished observation). Young et al. (14) showed that mature pre-IL-1P was not secreted from hamster fibroblasts that were stably transformed with pre-IL-1,i cDNA. Instead, large amounts of the precursor accumulated in the cytoplasm of the cell (14). Lomedico et al. (12) The processing of IL-1f3 has recently been investigated by using purified recombinant precursor as a substrate (5, 17).Hazuda et al. (5) showed that pre-IL-1f3, when added to intact human blood monocytes, was not cleaved or processed in any fashion, arguing against an extracellular site of processing. In another report, a potential pre-IL-1,8 cleavage activity was identified in a pelletable compartment of KG-1 cells, a neutrophil-like cell line. This enzymatic activity was able to generate IL-1 activity of similar size to authentic IL-1 from a partially purified pre-IL-1f3 substrate (17). However, the products were not sequenced and the site ofcleavage was not identified.In this report, we describe an in vitro processing system in which mature 17.

The quinazolinone and pyridol-pyrimidine classes of p38 MAP kinase inhibitors have a previously unseen degree of specificity for p38 over other MAP kinases. Comparison of the crystal structures of p38 bound to four different compounds shows that binding of the more specific molecules is characterized by a peptide flip between Met109 and Gly110. Gly110 is a residue specific to the alpha, beta and gamma isoforms of p38. The delta isoform and the other MAP kinases have bulkier residues in this position. These residues would likely make the peptide flip energetically unfavorable, thus explaining the selectivity of binding. To test this hypothesis, we constructed G110A and G110D mutants of p38 and measured the potency of several compounds against them. The results confirm that the selectivity of quinazolinones and pyridol-pyrimidines results from the presence of a glycine in position 110. This unique mode of binding may be exploited in the design of new p38 inhibitors.

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