Expression of functional recombinant human procathepsin B in mammalian cells

Cathepsin B has been implicated in numerous pathobiological processes. In order to study its interactions with other proteins implicated in these processes, quantities of functional recombinant cathepsin B are needed. Therefore, we expressed recombinant human procathepsin B in mammalian cells (BSC-1 monkey kidney cells and HeLa human cervical carcinoma cells) using a vaccinia virus expression system. The recombinant human procathepsin B appeared to be authentic and expressed in its native conformation as indicated by: (1) N-terminal sequencing; (2) molecular size; (3) processing intracellularly to mature double-chain cathepsin B; (4) in vitro cleavage by pepsin to mature cathepsin B coincident with appearance of activity against a selective synthetic substrate; and (5) substrate/inhibitor profiles. This is the first report of the expression of functional recombinant human procathepsin B in mammalian cells. We also report a single-step immunoaffinity purification procedure for the isolation of electrophoretically pure proenzyme. By the methodologies described, human procathepsin B can now be obtained in high yield. This should facilitate studies of its interactions with protease inhibitors, other proteases, extracellular matrices, cell-surface proteins and biological substrates that may be of relevance to the pathobiological functions of this enzyme.

The extracellular domain of p185/neu is released from the surface of human breast carcinoma cells, SK-BR-3

The human breast carcinoma cell line SK-BR-3, expresses the neu oncogene product, p185, which is a receptor tyrosine kinase. Using a double monoclonal antibody capture enzyme-linked immunosorbent assay for p185, activity was detected in conditioned media from cultures of SK-BR-3 cells. Two monoclonal antibodies specific for the extracellular domain of p185/neu immunoprecipitated a protein with a molecular mass of approximately 105 kDa. p105 was further shown to compete with p185 for binding to monoclonal antibodies and pulse-chase experiments indicate that it was generated by post-translational processing. Peptide maps showed that p105 and p185 are related polypeptides. Since p105 is close to the predicted size for the extracellular domain of p185/neu, we propose that SK-BR-3 cells specifically process and release this portion of the receptor into the medium. The release of the extracellular domain may have implications in oncogenesis and its detection could prove useful as a cancer diagnostic.

The role of lipids in the function of the acetylcholine receptor

The composition of membranes containing acetylcholine receptor was altered in order to examine the possible role of lipids in receptor function. Polyethylene glycol was used to fuse AcChR-rich membranes with an excess of lipid vesicles of defined composition. By this procedure, the cholesterol composition was reduced to as little as 20% of that found in native membranes. Using a T1+ flux assay it was possible to measure receptor function in such altered membrane environments. The apparent Kd for carbamylcholine was found to decrease as the cholesterol content was reduced. This result was confirmed by measuring the agonist-induced fluorescence change of the covalently attached probe, 4-[N-(iodoacetoxy)-ethyl-N-methyl]amino-7-nitrobenz-2-oxa-1,3-diaz ole. When the phospholipid composition was manipulated by membrane fusion, ion flux was found to be optimal when the lipid composition resembled that of native receptor membranes. These results indicate that membrane lipids potentially play a role in the regulation of acetylcholine receptor function.

Localization of the ATP binding site on alpha-tubulin

The binding site for ATP to tubulin was established by use of the photoaffinity label [gamma-32P]N3ATP. Photolysis of the analog in the presence of tubulin resulted in covalent modification of the protein as revealed by autoradiography of electropherograms. Scanning the autoradiograms showed that the ATP analog was bound mainly to the alpha subunit of the tubulin dimer; the alpha subunit was two to three times more radioactive than was the beta subunit. The location of a particular site on the alpha subunit was further defined by peptide maps. The alpha and beta subunits from affinity-labeled tubulin were separated and digested with Staphylococcus protease. Radioactivity was found predominantly in one peptide band from the alpha subunit. The location of the [gamma-32P]N3ATP binding site on the alpha subunit distinguishes it from the previously known exchangeable GTP binding site which is on the beta subunit. Moreover, excess GTP did not compete with [gamma-32P]N3ATP binding. The ATP binding site is distinct from the nonexchangeable GTP binding site. The GTP content of tubulin was the same after dialysis in 0.5 mM ATP as it was following dialysis against ATP-free buffer. Proof that the binding site for [gamma-32P]N3ATP is the same as that for ATP was obtained by competition experiments. In the presence of ATP, photolysis of the affinity analog did not label the alpha subunit preferentially.

Effect of ATP on the kinetics of microtubule assembly

We investigated the role of ATP in the assembly of microtubules. Tubulin, prepared by chromatography on DEAE-cellulose, was nearly devoid of nucleoside diphosphokinase activity. ATP induced assembly in such preparations for a single assembly/disassembly cycle; then further assembly could not be induced by ATP unless the system was supplemented with additional GTP. This suggests that the E-site must contain GTP for polymerization and ATP interacts at a different site on tubulin. Although tubulin can be assembled into microtubules in 1.0 mM GTP, the inclusion of 0.2 mM ATP along with the GTP increases the rate and extent of assembly. The enhancement increased with increasing ATP concentrations. The inclusion of 0.2 mM ATP reduced the critical concentration for tubulin assembly from 1.5 to 0.9 mg/ml. Analysis of assembly rate versus protein concentration suggested that ATP also affects nucleation. Aggregates of tubulin rings formed by warming tubulin in the presence of 1.0 mM ATP and 5.0 mM Mg2+ were capable of initiating assembly in a solution of tubulin which was not able to polymerize. Furthermore, the extent of microtubule formation was dependent on the concentration of aggregated rings added to the solution. We propose that ATP interacts with tubulin at a binding site that is distinct from the N- and E-sites that bind GTP. A function of ATP binding is to stimulate the formation of tubulin rings as nucleation centers for polymerization.

ATP-induced aggregates of tubulin rings

Aggregates of double-walled rings (46 nm diameter) were formed upon warming (37 degrees C) ion exchange purified bovine brain tubulin (1.5 to 2.0 mg/ml) in the presence of 1.0 mM ATP and 5.0 mM Mg2+. The formation of aggregated rings was blocked by GDP (1.0 mM), but when GTP (1.0 mM) was added subsequently, the block was overcome. Warming in the presence of ATP and GTP simultaneously produced a mixture of microtubules and aggregated rings. The tubulin preparations used were demonstrated to be devoid of transphosphorylase, and it is therefore clear that the action of ATP in the induction of aggregated rings was not mediated by transphosphorylation. These results are consistent with an interaction of nucleotide with tubulin at a site distinct from the previously described N- and E-sites.