However, entropy penalty itself cannot completely account for the significant loss of function in AcrBP223G, since the P224G mutation had no obvious effect on protein function. P223 could be serving as a wedge at the tip of the loop, which prevented the loop from slipping out of the tunnel once the trimer formed. A random mutagenesis study on a close homologue of AcrB, MexB in Pseudomonas aeruginosa, has lead to the identification of a functionally defective mutant with a point mutation in the loop, G220S. G220 is another conserved residue in the loop. The authors found that the mutant expressed at a level similar to that of WT MexB, and speculated that the G220S mutation may have hindered the insertion of the loop into the tunnel and thus decreased efficiency of MexB trimer formation through changing the secondary structure of the loop and causing steric problems. The position of the corresponding G220 in AcrB structure locates right at the kink in the loop, which further confirmed the importance of the kink in stabilizing the trimer structure. The loop locates right in between the TolCdocking domain and pore-forming domain of AcrB. There is also a possibility that in P223G mutation, transport-dependent conformational change could have been affected, which may further contribute to the observed drastic decrease of activity. From an evolutionary perspective, protein oligomerization offers clear functional advantages including enhanced structural DAPT scaffolding to support and regulate function, increased sensitivity to evolutionary pressure, and improved stability. However, the exact mechanisms by which proteins assemble into oligomers remain poorly understood. Here we used a homotrimeric membrane protein, AcrB, as a model system and investigated the connection between the oligomer stability and protein activity. We found the mutation of a residue critical to inter-subunit interaction “loosened” the AcrB trimer and thus drastically decreased the transport activity of the efflux pump. When tightened using an inter-subunit disulfide bond, the activity of the mutant improved dramatically. In addition, our result showed that during the trimerization of AcrB, the long protruding loop remained rigid, which suggested that its binding partners in the neighboring subunit underwent conformational adjustment to form a tunnel to accommodate the loop. The endoparasitic phylum Acanthocephala Kohlreuther, 1771 consists of about 1,150 species, belonging to 125 genera and 19 families. They are characterized by an evertable proboscis as the attachment organ, sexual dimorphism, males with cement glands and an uterine bell in females. Unique is the syndermatic tegument, placing the acanthocephalans, also confirmed by molecular studies, sister to the Rotifera. Recent classifications distinguish the four classes Archiacanthocephala, Eoacanthocephala, Palaeacanthocephala and Polyacanthocephala, with a majority of 62.7% of the species primarily infecting aquatic hosts. Around 57% species of the Acanthocephala belong to the Palaeacanthocephala with the two orders Echinorhynchida and Polymorphida.