PhD thesis defense in biological sciences by Hala KASMO

Decoupled Metal Transport in Caulobacter vibrioides: Distinct Roles of P1B-type ATPases in Copper Delivery and Zinc Homeostasis

Candidate

Hala KASMO

Promoter

Prof. Jean-Yves MATROULE, UNamur, Department of biology, Research Unit in Biology of Microorganisms (URBM)

Jury

• Prof. Xavier DE BOLLE (UNamur), Président
• Prof. Jean-Yves MATROULE (UNamur), Secrétaire
• Dr Françoise JACOB-DUBUISSON (Institut Pasteur Lille)
• Prof. Hans-Georg KOCH (University of Freiburg)
• Prof. Jean-François COLLET (UCLouvain)

Summary

Copper (Cu) is indispensable for bacterial respiration but toxic in excess. Most bacteria handle Cu detoxification using CopA-type P1B-1 ATPases and CopZ chaperones. However, we found out that Caulobacter vibrioides lacks both, indicating alternative Cu-handling strategies. Among its two P1B-type ATPases, one (P1B-1 subclass) is specifically required for maturation of the cbb₃-type cytochrome c oxidase (Cox), delivering Cu for enzymatic function rather than detoxification. A key finding is the identification of a TonB-dependent outer membrane transporter required for aa₃-Cox activity, revealing a novel bacterial Cu delivery pathway. We further demonstrated that cbb₃-Cox is upregulated under microaerobic conditions, likely resembling environments such as solid media where oxygen diffusion is limited. In contrast, under normoxic conditions, the expression and activity of cbb₃-Cox decrease, while aa₃-Cox becomes the primary terminal oxidase. We showed that C. vibrioides lacks P1B-2 ATPases, which are usually involved in Zinc (Zn²⁺) detoxification, unlike other bacteria like E. coli that have ZntA. Instead, C. vibrioides encodes a P1B-4 subclass ATPase that emerges as the likely mediator of Zn²⁺ homeostasis: disruption of this ATPase causes Zn2+ sensitivity and supports its role in Zn²⁺ handling. Further studies are needed to define its mechanism. Together, the results show that C. vibrioides recruits each P1B-type ATPase via separate, substrate-specific systems: one specializing in Cu delivery for respiration and another involved in Zn homeostasis. Overall, this study highlights the organism-specific specialization of P1B-type ATPases and provides insight into minimalist and highly adapted metal transport architectures within bacteria.