<p>Type 4 pili (T4P) are fibrous appendages found on the surfaces of a wide range of bacteria. They are used for adherence to biotic and abiotic surfaces, twitching motility, and biofilm formation. Despite their ubiquitous distribution, identifying the core components required for T4P expression has been difficult due to conflicting data about the functions of orthologous components from the most common model organisms, <em>Neisseria</em> and <em>Pseudomonas</em>. By inactivating the retraction component of pilus function, genes essential for T4P assembly versus disassembly were discriminated in <em>P. aeruginosa</em>. In contradiction to data from the <em>Neisseria </em>system<em>,</em> we found that components of the inner membrane sub-complex consisting of PilN/O/P are not essential for surface pilus expression, while the highly conserved inner membrane protein, PilC is essential. The current model of T4P biogenesis suggests that PilC coordinates the activity of cytoplasmic extension (PilB) and retraction (PilT) ATPases via their interaction with its two large cytoplasmic domains. Hydrolysis of ATP by PilB or PilT is proposed to induce domain movements in PilC, resulting in the addition or removal of single pilin subunits from the base of the pilus. Using<em> </em><em>in vitro</em> co-affinity purification we showed that PilB is a potential interaction partner of the N-terminal cytoplasmic domain of PilC. Also, mutagenesis of the C-terminal cytoplasmic domain of PilC produced mutant proteins with a reduced capacity to support twitching motility, suggesting impairment of PilC-PilT interactions. The indispensability of PilC and its potential interactions with the ATPases PilB and PilT suggest that it is a core element required for function of the T4P system of <em>P. aeruginosa</em>.</p> / Master of Science (MSc)