Rickettsia Actin-Based Motility Occurs in Distinct Phases Mediated by Different Actin Nucleators

Abstract

Summary Many intracellular bacterial pathogens undergo actin-based motility to promote cell-cell spread during infection [1]. For each pathogen, motility was assumed to be driven by a single actin polymerization pathway. Curiously, spotted-fever-group Rickettsia differ from other pathogens in possessing two actin polymerizing proteins. RickA, an activator of the host Arp2/3 complex, was initially proposed to drive motility [2, 3]. Sca2, a mimic of host formins [4, 5], was later shown to be required for motility [6]. Whether and how their activities are coordinated has remained unclear. Here, we show that each protein directs an independent mode of Rickettsia parkeri motility at different times during infection. Early after invasion, motility is slow and meandering, generating short, curved actin tails that are enriched with Arp2/3 complex and cofilin. Early motility requires RickA and Arp2/3 complex, and is correlated with transient RickA localization to the bacterial pole. Later in infection, motility is faster and directionally persistent, resulting in long, straight actin tails. Late motility is independent of Arp2/3 complex and RickA, and requires Sca2, which accumulates at the bacterial pole. Both motility pathways facilitate cell-to-cell spread. The ability to exploit two actin assembly pathways may allow Rickettsia to establish an intracellular niche and spread between diverse cells throughout a prolonged infection.

The WH2 Domain and Actin Nucleation: Necessary but Insufficient

“…Similar to Listeria monocytogenes , SFGR use an actin comet tail mechanism for motility. Yet, unlike Listeria and other pathogens that use actin-driven motility, SFGR use two independent actin polymerization pathways [38]. Thus, early after infection motility is slow, driven by short and curved comet tails that are formed by RickA, an NPF expressed on the surface of the bacterium that recruits and activates the host cell Arp2/3 complex.…”

Secretome of obligate intracellularRickettsia

“…However, very recent research on R . parkeri demonstrated that RickA and Sca2 each direct independent modes of ABM, with RickA generating short, curved actin tails early during infection and Sca2 generating long, straight actin tails later in infection (Reed et al ., 2014) . This is partially consistent with a previous suggestion that RickA targets actin for the facilitation of host cell entry (Haglund et al ., 2010) , but reveals that RickA-induced HAP, like that of Sca2, can facilitate intercellular spread in some species.…”

Intracellular Bacteria Find the Right Motion

“…These proteins act processively on barbed ends in which they sit during the entire polymerization process. Interestingly, while Listeria and Shigella use Arp2/3, the intracellular pathogens Rickettsia early in infection use Arp2/3 and later on switch to a different mechanism, expressing proteins that mimic formins to promote actin-based motility (Welch et al, 1998;Suzuki et al, 1998;Gouin et al, 2004;Reed et al, 2014) (see below and Figure 1). …”

“…In the case of Shigella, the outer-membrane protein IcsA/VirG (Bernardini et al, 1989) recruits N-WASP (Suzuki et al, 1998), which in turn recruits and activates Arp2/3 (Figure 1). In Rickettsia, early during infection, the protein RickA, which displays a VCA domain, mimics WASP family proteins and recruits Arp2/3 (Gouin et al, 2004;Reed et al, 2014). Later, the surface protein Sca2 acts as a formin (Reed et al, 2014).…”

“…In Rickettsia, early during infection, the protein RickA, which displays a VCA domain, mimics WASP family proteins and recruits Arp2/3 (Gouin et al, 2004;Reed et al, 2014). Later, the surface protein Sca2 acts as a formin (Reed et al, 2014). As Sca2, BimA proteins belong to a family of bacterial proteins called ''autotransporters'' because they have the capacity to insert themselves in the outer membrane and then display their N-terminal parts on the outside of the bacterium.…”