Umami lasts longer than other tastes since it involves a deeper, more lasting level of cell signalling. As described, salty taste involves what is actually on either side of tongue cells at a given moment, while umami flavor persists well after glutamate has been perceived. This allows it to last as long as the cell signalling cascade.
Work Step by Step
The key here is the hint that umami involves a GPCR-- review the action of these receptors compared to the shorter-term action described for salty taste.