"what is called" vs ", what is called, "

[Rei]

Hi,
1 My mother is what is called a walking dictionary.
2 My mother is, what is called, a walking dictionary.

Are these both acceptable?
If so, which is more natural? Which do you prefer?
If so, what is the difference?

Thanks in advance!

[Mister Micawber]

.
Only #1 is acceptable.
.
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Native English teacher at Mister Micawber's