Lactococcus lactis, an Attractive Cell Factory for the Expression of Functional Membrane Proteins

Abstract

Membrane proteins play key roles in most crucial cellular processes, ranging from cell-to-cell communication to signaling processes. Despite recent improvements, the expression of functionally folded membrane proteins in sufficient amounts for functional and structural characterization remains a challenge. Indeed, it is still difficult to predict whether a protein can be overproduced in a functional state in some expression system(s), though studies of high-throughput screens have been published in recent years. Prokaryotic expression systems present several advantages over eukaryotic ones. Among them, Lactococcus lactis (L. lactis) has emerged in the last two decades as a good alternative expression system to E. coli. The purpose of this chapter is to describe L. lactis and its tightly inducible system, NICE, for the effective expression of membrane proteins from both prokaryotic and eukaryotic origins.

Keywords: Lactococcus lactis; NICE system; membrane proteins.

Figure 1

The Nisin-Controlled Gene Expression (NICE) system in L. lactis. After the detection of nisin by the membrane-located sensor protein (NisK) ①, this histidine protein kinase autophosphorylates and transfers its phosphate group to activate the cytoplasmic response regulator NisR ②. Activated NisR ③ subsequently induces transcription controlled by the PnisA promoter ④. After transduction, ⑤ and depending on the presence or absence of the corresponding targeting signals, the protein is either expressed into the cytoplasm or the membrane, or secreted into the external medium ⑥. B. chr: bacterial chromosome. Adapted from [12,25].

Figure 2

Comparison of MPs expressed in L. lactis using the NICE system depending on their origin: L. lactis, other bacteria, or eukaryotic cells.

Figure 3

Influence and relationship between origins of MPs expressed in L. lactis.

Figure 4

Influence of number of TM helices on expression of MPs expressed in L. lactis. (a) On expression of MPs from L. lactis. (b) On expression of MPs from other bacteria. (c) On expression of eukaryotic MPs.

Figure 5

Relationship between function (transporters in red, mitochondrial proteins in green, and other functions in black), size, and topology of MPs expressed in L. lactis.