Secretion of functional human enzymes by Tetrahymena thermophila

Background

The non-pathogenic ciliate Tetrahymena thermophila is one of the best-characterized unicellular eucaryotes used in various research fields. Previous work has shown that this unicellular organism provides many biological features to become a high-quality expression system, like multiplying to high cell densities with short generation times in bioreactors. In addition, the expression of surface antigens from the malaria parasite Plasmodium falciparum and the ciliate Ichthyophthirius multifiliis suggests that T. thermophila might play an important role in vaccine development. However, the expression of functional mammalian or human enzymes remains so far to be seen.

Results

We have been able to express a human enzyme in T. thermophila using expression modules that encode a fusion protein consisting of the endogenous phospholipase A₁ precursor and mature human DNaseI. The recombinant human enzyme is active, indicating that also disulfide bridges are correctly formed. Furthermore, a detailed N-glycan structure of the recombinant enzyme is presented, illustrating a very consistent glycosylation pattern.

Conclusion

The ciliate expression system has the potential to become an excellent expression system. However, additional optimisation steps including host strain improvement as wells as measures to increase the yield of expression are necessary to be able to provide an alternative to the common E. coli and yeast-based systems as well as to transformed mammalian cell lines.

The MICAL proteins and rab1: a possible link to the cytoskeleton?

The small GTPase rab1 plays a role in vesicle trafficking between ER and the Golgi complex. Recently, MICAL-1 was identified as new rab1 interacting protein. In this study, we show an interaction between two additional members of the MICAL family and rab1 in a yeast two-hybrid approach and GST pulldown experiments. We present data about the previously uncharacterized MICAL-3 concerning tissue distribution, size, and cellular localization. Furthermore, we investigated the connection between MICAL proteins and the cytoskeleton. Using the microtubule depolymerizing drug nocodazole we detected a link between MICAL-1 and -3, and the microtubule cytoskeleton.

Identification and characterization of Iporin as a novel interaction partner for rab1

Background

The small GTPase rab1a and its isoform rab1b are essential regulating components in the vesicle transport between the ER and the Golgi apparatus. Rab1 is thought to act as a molecular switch and can change between an active GTP-bound and an inactive GDP-bound conformation. To elucidate the function of rab1, several approaches have been established to isolate effector proteins, which interact with the activated conformation of rab1. To date p115, GM130, golgin-84 and MICAL have been identified as direct interacting partners. Together with rab1, these molecules are components of a protein complex, which mediates and regulates intracellular vesicle transport.

Results

Here, we report the characterization of Iporin, which is similar to KIAA0375 as a novel rab1-interacting protein. It was initially identified by yeast two-hybrid screening experiments with the active mutant of rab1b (rab1b Q67R) as bait. Iporin contains a SH3 domain and two polyproline stretches, which are known to play a role in protein/protein interactions. In addition, Iporin encloses a RUN domain, which seems to be a major part of the rab1binding domain (R1BD). Iporin is ubiquitously expressed and immunofluorescence staining displays a cytosolic punctual distribution. Interestingly, we also show that Iporin interacts with another rab1 interacting partner, the GM130 protein.

Conclusion

Our results demonstrate that Iporin is a potential new interacting partner of rab1. Iporin is different from already identified rab1 interacting proteins concerning protein structure and cellular localization. We conclude that Iporin might function as a link between the targeting of ER derived vesicles, triggered by the rab1 GTPase and a signaling pathway regulated by molecules containing SH3 and/or poly-proline regions. The characterization of this novel intermolecular relation could help to elucidate how vesicles find their way from ER to the Golgi apparatus.

MICAL-1 isoforms, novel rab1 interacting proteins

Rab1 GTPases participate in regulating the vesicular transport of ER-Golgi compartments. Recently, GM130, p115, and Golgin-84 were identified as effectors of the active conformation of rab1. Here, we describe a novel protein, MICAL-1b, a splice variant of the MICAL-1a protein. Using the yeast two-hybrid system, we showed that it specifically interacts with rab1 in a nucleotide-dependent manner. The interaction was confirmed by GST pulldown experiments. Cell fractionation revealed that in contrast to the mainly membrane-associated rab1 effector GM130, MICAL-1 displays a predominantly cytosolic localization. We mapped the rab1 interacting domain to the C-terminus of MICAL-1, which also mediates binding to the intermediate filament vimentin. Therefore, the interaction of MICAL-1 and rab1 might provide a link between the Golgi apparatus and the intermediate filament cytoskeleton. We suggest that MICAL-1 isoforms with their multidomain structure are novel rab1 interacting proteins that function as scaffold proteins connecting different components in the cell.

Bicaudal-D regulates COPI-independent Golgi-ER transport by recruiting the dynein-dynactin motor complex

The small GTPase Rab6a is involved in the regulation of membrane traffic from the Golgi apparatus towards the endoplasmic reticulum (ER) in a coat complex coatomer protein I (COPI)-independent pathway. Here, we used a yeast two-hybrid approach to identify binding partners of Rab6a. In particular, we identified the dynein-dynactin-binding protein Bicaudal-D1 (BICD1), one of the two mammalian homologues of Drosophila Bicaudal-D. BICD1 and BICD2 colocalize with Rab6a on the trans-Golgi network (TGN) and on cytoplasmic vesicles, and associate with Golgi membranes in a Rab6-dependent manner. Overexpression of BICD1 enhances the recruitment of dynein-dynactin to Rab6a-containing vesicles. Conversely, overexpression of the carboxy-terminal domain of BICD, which can interact with Rab6a but not with cytoplasmic dynein, inhibits microtubule minus-end-directed movement of green fluorescent protein (GFP)-Rab6a vesicles and induces an accumulation of Rab6a and COPI-independent ER cargo in peripheral structures. These data suggest that coordinated action between Rab6a, BICD and the dynein-dynactin complex controls COPI-independent Golgi-ER transport.

The Golgi matrix protein GM130: a specific interacting partner of the small GTPase rab1b

To detect specific partners of the small Golgi-localized GTPase rab1b we generated rab1b mutants and used them as bait proteins in yeast two-hybrid screens. We isolated several specifically interacting clones. Two of them encode large protein fragments highly homologous to rat GM130 and to human Golgin95. The full-length human GM130 cDNA was cloned and its interaction with rab1b was characterized in detail by yeast two-hybrid and in vitro binding assays. Here we report for the first time that the rab1b protein interacts specifically with GM130 in a GTP-dependent manner and therefore needs the hypervariable regions of the N- and C-termini. We mapped the rab1b binding site of GM130 and provide evidence that it is different to the previously described p115 and Grasp65 binding sites of the GM130 protein.

Inactive and active mutants of rab1b are not tightly integrated into target membranes

The rab1b GTPase is localized on the ER and cis-Golgi membranes and involved in early exocytotic membrane traffic processes. To analyze the influence of rab1b conformation on specific membrane targeting processes three point mutants simulating permanently inactive or active rab1b proteins were constructed and expressed in BHK cells. To increase the expression of the growth inhibiting rab1b S22N and rab1b N121I mutants we co-expressed the Mss4 protein and observed an elevated expression of the inactive rab1b mutants. Surprisingly, only the rab1b wild-type protein shows the correct intracellular localization, and a tight membrane association. We conclude that the targeting process of rab1b depends predominantly on GDP/GTP exchange.