Conrad A. King

Locomotion and Phenotypic Transformation of the Amoeboflagellate Naegleria gruberi at the Water–Air Interface

Abstract The protozoon Naegleria gruberi is able to carry out amoeboid locomotion at the water–air interface in a manner indistinguishable from that exhibited on solid substrata with the production of focal contacts and associated filopodia. The speed of locomotion at this interface can be modulated by changes in electrolyte concentrations; these speed changes are identical to those observed at a water-glass interface. The nature of the water–air interface is discussed leading to the hypothesis that surface tension alone could provide suitable properties for the adhesion and translocation of amoebae at this interface without necessitating specific, absorbed molecules. The temporary swimming flagellate stage of Naegleria is able to dock at the interface, make stable adhesions to it, and revert to the amoeboid phenotype. Conversely, amoebae resident at the water–air interface can transform to swimming flagellates and escape into the bulk liquid phase. We report the presence of Naegleria amoebae in the surface microlayers of natural ponds; thus, in freshwater bodies there may be active shuttling of Naegleria amoebae from the benthos to the surface microlayers by means of the non-feeding, swimming flagellate phenotype. The public health implication of this behaviour in the case of the pathogenic relative, Naegleria fowleri, is discussed.

Actin‐Based Motility in the Net Slime Mould Labyrinthula: Evidence for the Role of Myosin in Gliding Movement

Abstract. In contrast to crawling movement (e.g. in amoebae and tissue cells) the other major class of substratum‐associated motility in eukaryotes, gliding, has received relatively little attention. The net slime mold Labyrinthula provides a useful laboratory model for studying this process since it exhibits a particular kind of gliding in its plasmodial stage. Here nucleated spindle cells glide along self‐established cytoplasmic trackways in a predominantly unidirectional manner, at 1–2 μm/s. These trackways, upon which gliding is dependent, are held by filopodial tethers some distance off the well‐developed reticulopodial mesh anchoring the plasmodium onto the substratum. Reflection interference microscopy resolves this matrix in live plasmodia. The axially disposed cytoskeletal elements of the trackways are revealed by rhodamine‐labelled phalloidin to be rich in F‐actin. A weft of peripheral, rapidly extending filopodia (50 μm/min) typifies the expanding regions of the plasmodium. Here spindle cells are recruited before emigrating into newly differentiated trackways. Immunoblotting whole plasmodia or a sucrose‐soluble cytoplasmic extract reveals a single actin‐positive band of Mr 48 kDa. Polyclonal antibodies to two distinct myosin peptide sequences identify a single myosin HC (Mr 96 kDa) in immunoblots. Gliding was reversibly blocked by 10 mM 2,3‐butanedione‐2‐monoxime, a myosin ATPase inhibitor, but it was insensitive to the actin‐binding drugs cytochalasin D and phalloidin. We suggest that the force (>50 pN) for gliding motility results from interaction of myosin molecules, associated with the spindle cells, with trackway F‐actin via the bothrosomes.