HRP transport. Translocation experiments were performed using DIBs
that contained 10 mol% of negatively-charged phosphatidylglycerol
(PG) and 90% PC lipids in one or both leaflets. Unlike Pep-1, D9-NBD
based translocation of HRP showed no significant dependence on
charge distribution in the DIBs (Fig. 4). The addition of a voltage bias
did not increase translocation across an asymmetric DIB (Fig. 4). The
lack of dependence of D9-NBD facilitated HRP transport on either voltage
or membrane asymmetry suggests that D9 and perhaps other
guanidinium-rich carriers may function by a different mechanism
than Pep-1.
To explore these differences further, we examined the relationship
between complex formation and carrier-mediated transport using D9-
NBD and Pep-1. In these experiments, the cargo and carrier were placed
either in the same droplet or in opposite droplets. For Pep-1, HRP transport
was not observed in either case. However, D9-NBD showed a remarkable
increase in HRP transport when placed on the opposite side
of the DIB from HRP (Fig. 5). Further, DLS measurements indicate that
D9-NBD does not adsorb to HRP when they are incubated together
(Table S2). Two important ideas emerge from these results. First, interactions
between D9 and HRP, such as an incubation period, are not a
prerequisite for translocation. This contrasts with Pep-1 where several
literature reports emphasize the importance of both incubation and
the peptide to cargo molar ratio. Second, transport is enhanced when
the cargo and carrier are separated. Guanidinium-rich peptides have already
been shown to “reach across” vesicle membranes to extract bulky
anions [9]. D9-NBD is also capable of extracting bulky anions, in this case
carboxyfluorescein, from vesicles in a concentration-dependent manner
(Fig. 5, inset). Our data suggests that this idea might be extended to
larger entities such as entire proteins. Following several of the DIB experiments
presented here, we imaged the capture droplet by fluorescence
microscopy to measure how much D9-NBD crossed over after
translocation.
However, very little D9-NBD was detected in the capture droplets
(except where D9-NBD and HRP were in separate droplets) (Fig. S8).
This does not necessarily indicate that D9 cannot cross the DIB, but at
least it does not rapidly equilibrate across the membrane. Models like
carpet, toroidal pore and barrel stave are insufficient to explain the
transport of molecular cargo observed in our experiments with Pep-1
and D9 as carriers [16,35–36]. We note that both Pep-1 [31] and D9
form pores in DIB membranes in the absence and presence of cargo
(Fig. S7). This suggests that the requirement for complex formation
(as with Pep-1) [5] may not be related to the formation of pores in the
membrane. If so, perhaps translocation does not take place by passing
through carrier-lined pores but instead by some other means. For example,
the hydrophobic regions of either D9 or Pep-1 could interact with
the hydrophobic core of the membrane while the positive regions neutralize
negatively-charged patches on the cargo. The lysine-rich Pep-1