Índice de Serviciabilidad Presente (PSI)

C a th ep s in s B, H, L and S are the most well ch a r a c t e r is e d cysteine p r ot e in a se s to date. These enzymes have been loc ali sed to mo st endocytic s tru ctu re s, from endosomes to lysosomes, in which pr o te in degradation occ urs (Sakai et al, 1991 ; Ma ciew icz et al, 1991 ; Saku et al, 1993; Reise et al, 1996, Bevec et al, 1996). Cysteine prote ina ses d iffer from the aspartic ca t h ep s in s in that they are able to function as both e n d o p r o t e i n a s e s and ex o p r o t ei n as es . Cysteine cathepsins are thought to have a func tiona l pH t h re s h o l d of 5.5, above which they fun ctio n as e x o p r o t e i n a se s , and below whi ch they act as endo pr ot ein ase s (Rodriguez & D im en t, 1995). This i m p li es that the function of these enzymes is de pe nd en t on the proc ess ing c o m p a r t m e n t in which they are acting. The pH gra die nt that exists across A PC implies that in earlier co m par tm en ts, these en zym es behav e as e x o p r o t e in a se s , while acting as end op ro te in as es in later c o m p ar tm e n ts .

1.5.2.1 Cathepsin B

A ub iq u it o us , lysosomal hous e-k eep in g enzyme, cat he ps in B belongs to the pa p a i n su perfamily of proteinases (Barrett & Kirsce, 1981). C at hep si n B is

a p ot en t end o-c ar b o xy p e tid as e, but only displays li m it ed e nd o pr ot ei na se activity (M aci ew ic z et al, 1991; van Noort et al, 1991; Reise et al, 1996). As with other lysosmal pro teina ses, such as cathep sin D, Cathep sin B has been i m p l i c a t e d in a wide range of physiol ogi cal p r oce ss es in clu di ng bone re so rp ti on (Délaissé et al, 1991), gen eration of renin fro m the pro -e n zy me form (Wang et al, 1990), thyroxin proc ess in g (Dunn et al, 1991) and prote in tu rn o v e r in general (Shaw & Dean, 1980).

Ca th ep s in B is synthesised in the same way as other c at h ep s in pr oein ases; as a p r e -p r o -e n z y m e on the surface of the RER (Barrett & Kirscke, 1981). Loss of the pr e-enzyme signal sequence in the ER is fo llo we d by ph o s p h o ry la t i o n of the mannose residues on the car bo h yd ra te groups of the p r o- en zy m e in the golgi. As with cathe ps in D and other lysos oma l enzymes, this mar ks the pro-cath eps in B for recognition, and sign alin g to the l y s os om es /l at e endosomal structures by the m a n n o s e -6 - p h o s p h a t e rec ept or pathway. A mature, single chain form of cat h ep si n B results from au to cat al ys is of the pr o-enzyme in such c o m p ar tm e n ts , fo llo we d by the sequ ent ial re moval of three di peptides (Rowan et al, 1992; Rowan et al,

1993). The 30kD single chain mature enzyme can di m eri se in the lysosomes or late e n d os om e s to form an enzyme cons ist in g of a 25kD heavy chain and a 5kD light chain. This is ach ieved by cleavages be tw een residues 47 and 50, with the loss of the co r res po nd in g int ers pac ed dip epti de. Resi du es 50- 254 then form the heavy chain, with residues 1-47 co m p r is i n g the light chain (Mach et al, 1992). Mature catheps in B is disc shaped in structure, with a m ar ke d ind entation r epr es ent ing the cleft fo rm in g the active site. The key residues conta ined within the active site, both an absolute r e q u i re m e n t for catalysis, are a cysteine at re sidue 29 and a hist idi ne at po sit io n 199 (Musil et al, 1991 ; Jia et al, 1995). A ct i v at i o n by thiol groups is a r e q u i r e m e n t for catheps in B fu nction (M ar te l l -P el le ti er et al, 1990).

1.5.2.2 C athepsin H

A po te n t a m in op ep ti d as e with, as cathe ps in s B and L, weak e n d op ep t i d as e activity, cat h ep si n H is also a ubi qu it ou s lysosomal enzym e (Lafu se et al,

1995). Little is known about its role in antigen p r o c e s s in g and pr es en ta ti o n. In vitro degradation studies have shown ca th eps in H to be

unable to degrade any part of the a / p / I i chain trimeric co mp lex in a cell-

free syst em (Reise et al, 1996).

1.5.2.3 C athepsin L

An ot h er of the lysosomal cysteine pro teina ses, cathe ps in L has been im pl ic at ed in the degradation of inva ria nt chain (Ogrinc et al, 1993, Bevec

et al, 1996). There is evidence to suggest that ca th eps in L is re g u la te d in its li pro te o lyt ic capacity by a degra dat ion pr odu ct of the p41 form of in v ar ia n t chain (Bevec et al, 1996) in that once re le as ed from the p41 li, the li f ra gm en t binds the catheps in L, thus p re ve n ti n g any further p ro te o ly t i ca l l y activity. Cathepsins S and B are not in h ib it ed by such li- der ived peptides. This is the first evi dence to suggest the p re sen ce of factors re sp o n s ib le for the differential regulation of cys teine p ro tei na ses . Such re g u l a t io n makes sense in light of the milieu of cys teine pr ot e in a se s p re sen t t h r o u g h o u t the endocytic pathw ay of APC.

1.5.2.4 Cathepsin S

Or ig ina lly cloned from human al veolar m ac ro pha ges , this cysteine pr o te in a se has been id entified in B cells, m acr op hag es and bone marrow- deri ved m u rin e DC (Shi et al 1994; Mo rton et al, 1995; Reise et al, 1996). C at he ps i n S m ed iat ed degradation of inva ria nt chain (li) appears to be a r e q u i re m e n t for MHC Il-ag as sociation in B cells (Reise et al, 1996).

1.6 E v id en ce for a role in an tigen p ro cessin g for the