**3.2 Structural studies of the soluble Lts, Slt, SLtB1 and SltB3 of** *P. aeruginosa*

X-ray crystal structures of Slt in its apo form as well as in complex with various synthetic PG substrates and reaction products demonstrated that this Lt has both exolytic and endolytic activity [23]. It is a donut shaped protein like Slt of *E. coli*. Notably, it is only after the binding of substrates that contain pentapeptide stems that it can exhibit endolytic activity due to a conformational change of the protein on substrate binding. A movie of this rearrangement is available in the supplementary material of reference [23]. Additional studies suggest protein–protein interactions with inner membrane PBPs are also important [26].

SltB1 [22] and SltB3 [24] have also been studied using x-ray crystallography. SLtB1 protein structures suggest that the protein forms a so-called "catenane" homodimeric structure in which the active sites face one another and are thus completely occluded. It is speculated that this soluble dimer may represent a form of activity regulation [22]. SltB3 is an exolytic enzyme with four distinct enzymatic domains within the donut shaped annular protein [24]. SltB3 can recognize PG substrates that are 4–20 sugars in length. These PG chains thread through the annular structure during catalysis.

#### **3.3 Structural studies of the endolytic Lt, MltF**

X-ray crystal structures of a solubilized MltF [21] show that this Lt binds a tetrapeptide stem of the substrate in an allosteric domain. Binding causes a large conformational change that leads to enzyme activation. In the kinetic studies, this solubilized membrane had very low activity with any of the 4 synthetic substrates or the *P. aeruginosa* sacculus. This raises some questions regarding the actual role of this Lt and whether the conformational changes are relevant when the protein is membrane bound.
