Biomedical Engineering Reference
In-Depth Information
Systematic deletion mutations of genes within the Magnetosome Island have
led to the identifi cation of several protein functions. The
mamK
gene encodes a
cytoskeletal fi brous actin-like protein MamK, while another gene unique to mag-
netic bacteria,
mamJ
, was also found within the Magnetosome Island. The deletion
mutants of each of these genes produced cells capable of producing magneto-
somes, but not in a chain formation; rather, they were randomly displaced through-
out the cell. Thus, it has been shown that MamK is a stabilizing fi lamentous
protein which runs alongside the magnetosomes, while MamJ interacts between
the magnetosome and MamK, so as to “ stick ” magnetosome “ beads ” to a MamK
“ string ” [72, 89] .
11.4.3.3
Mechanism
Today, a rich array of microbiological, physical, genetic and proteomic data are
available with which to propose a substantial mechanism of magnetosome forma-
tion. Initially, budding and invagination of the cytoplasmic membrane occurs,
during which vesicle formation the magnetosome membrane- specifi c proteins are
expressed or recruited to specifi c locations on the vesicle; this activation step is
thought to be associated with Mms24. In order to commence magnetite biomin-
eralization, large quantities of iron must be taken up by the cell from the environ-
ment. Generally, it is suggested that ferrous ions are transported, based on the
kinetics and the abundant ferrous ion transport proteins, although siderophores
have been observed in some magnetic bacteria [90] and the complete picture of
iron transport remains unknown. However, more recent transcriptome analyses
of iron transport protein expression have revealed that high- affi nity ferrous ion
transport occurs under iron-rich conditions, whereas siderophore ferric ion trans-
port dominates in a more depleted iron environment [91].
It is now known that the magnetosome membrane is not a true vesicle, but
remains attached to the cytoplasmic membrane. What is not known, however, is
whether the neck of the vesicle is open or plugged with proteins, and also whether
the iron ions are transported across the outer and inner membranes and then into
the magnetosome membrane, or are simply transported across the outer mem-
brane directly into the periplasm and into the neck of the magnetosome. The
accumulated ferrous ions within the magnetosomes are then thought to bind and
nucleate on Mms6 (and probably also 5 and 7) and begin magnetite formation
with partial oxidation (possibly by MamT) and pH regulation (possibly by Mam
N). Initial iron-storage phases and precursors such as ferritin have been proposed
[92], although there is no direct evidence of this, and it seems that the magnetite
is directly mineralized. Magnetosome formation occurs simultaneously in all of
the empty vesicles nucleating from similar locations within the vesicle [53, 71].
The crystal growth, size, and shape is then regulated by Mms6 (and probably also
5 and 7). The magnetosomes are aligned in chains by using MamJ to attach to the
fi brous MamK protein. The speed at which magnetite formation occurs has proved
to be highly variable, and appears to depend on iron and nutrient uptake. Indeed,
several studies in limited media (to inhibit growth) have provided formation times
in excess of 5 h [71, 92, 93], while large-scale fermentor growth has shown a slow
